The Maternal-Infant Microbiome Continuum: From Pregnancy Programming to Lifelong Health

1. Introduction

The period from conception through early infancy represents a critical window for human development, where environmental exposures can have lifelong consequences. Among these, the maternal microbiome comprising the complex communities of bacteria, archaea, fungi, and viruses residing in the gut, vagina, oral cavity, and mammary tissue, constitutes a primary microbial environment for the developing offspring [1,2,3]. This review posits the existence of a maternal-infant microbial continuum, a dynamic and interactive system where maternal microbial status directly influences pregnancy success, neonatal colonization, and the child’s long-term health trajectory [4,5,6].

Historically, research into maternal-fetal medicine and early-life development has underemphasized the role of microbial ecosystems. While the physiological adaptations of pregnancy (e.g., hormonal and immunological changes) are well-characterized, the concurrent and functionally significant remodeling of maternal microbiomes has only recently come into focus [1,7]. This gap is compounded by a historical underrepresentation of pregnant individuals in large-scale microbiome studies and methodological challenges, such as the heated debate surrounding the existence of a true placental microbiome [8,9]. Consequently, defining a “healthy” perinatal microbiome and translating associations into causal mechanisms remain significant challenges [10,11].

The imperative to study this continuum is underscored by robust epidemiological and mechanistic links between maternal dysbiosis, an dysbiosis in microbial communities and a spectrum of adverse outcomes [4,12]. For the mother, this includes obstetric complications like PTB, PE, and GDM [13,14,15]. For the offspring, the influence extends beyond birth, programming susceptibility to non-communicable diseases (NCDs) such as asthma, metabolic syndrome, and neurodevelopmental disorders [16,17,18]. This connection positions the maternal microbiome not merely as a bystander but as a modifiable target for prenatal intervention, offering a novel avenue for primary prevention [19,20].

This review will critically examine the evidence for this maternal-infant microbial continuum. First, we will describe the physiological and dysbiotic changes in key maternal niches (gut, vaginal, oral) during pregnancy [3,21,22]. Next, we will analyze the routes of vertical microbial transmission, critically assessing the evidence for in utero transfer and the profound impact of birth mode [2,23,24]. We will then trace the establishment and maturation of the infant microbiome in the first 1,000 days, focusing on its role in educating the immune system [25,26,27]. Subsequently, we will detail the mechanistic links between dysbiosis and specific diseases [28,29,30]. Finally, we will evaluate current and emerging microbiota-targeted interventions, concluding with future directions for research and clinical translation [10,31,32].

While previous reviews have examined specific aspects of the maternal microbiome such as pregnancy-related changes, vertical transmission, or infant colonization in isolation this review provides a comprehensive synthesis of the entire maternal-infant microbial continuum from preconception through the first 1,000 days after birth. We integrate mechanistic insights from animal models and human studies, critically evaluate the strength of associations between dysbiosis and disease, assess the evidence for various intervention strategies, and propose a practical framework for translating these findings into clinical practice. By examining the continuum as a unified system rather than discrete phases, we identify key windows of opportunity for intervention and highlight critical knowledge gaps requiring urgent research attention.

1.1. The Maternal Microbiome as the First Environment:

The maternal microbiome, encompassing the complex microbial communities colonizing sites such as the gut, vagina, oral cavity, and mammary tissue, is recognized as the primary microbial environment for the developing offspring [1,33]. During gestation, the maternal body undergoes significant hormonal, immunological, and metabolic adaptations to promote fetal development and prepare for parturition and lactation [3,7]. The maternal microbiota actively participates in and modulates these changes, contributing to homeostasis and supporting maternal health [3,34]. This complex ecosystem is pivotal in orchestrating the education and programming of the infant’s immune system, establishing the foundation for metabolic and general health that can persist for years [3,35,36].

1.2. Historical Underrepresentation of Women and Infants in Microbiome Research:

Despite the acknowledged importance of the gestation period for human health, studies specifically addressing the impact of the maternal microbiota on pregnancy progression and its modulation by maternal lifestyle are scarce [3,37]. Historically, research insights into the mechanisms linking microbial alterations to gestational complications have largely relied on observational studies or evidence derived from animal models, posing challenges for direct translation to the complex human context [3,10].

This gap is compounded by methodological controversies, most notably the heated debate surrounding the existence of a true placental microbiome. While early sequencing studies detected bacterial DNA in placental tissue, subsequent rigorous, contamination-controlled research has largely challenged the concept of a resident microbial community in healthy term pregnancies (Figure 1), and earlier signals potentially arising from low-level microbial invasion or technical artifact [9,38,39]. Resolving such controversies requires standardization in sampling, analysis, and data reporting methodologies [10,40].

2. Rationale for Studying Maternal–Infant Microbial Crosstalk:

The rationale for studying maternal–infant microbial crosstalk is underscored by the strong and persistent association between maternal dysbiosis and a wide array of adverse maternal and offspring health outcomes [4].

a)

Maternal and Obstetric Complications: Alterations in the maternal microbiota are critically linked to major adverse pregnancy outcomes, including Preterm Birth (PTB), Preeclampsia (PE), and Gestational Diabetes Mellitus (GDM). For instance, certain microbial features, such as the enrichment of the species Clostridium innocuum in the maternal gut during early pregnancy, have been identified as predictive biomarkers for a higher risk of PTB [13].

b)

Long-Term Offspring Disease Programming: The maternal microbial status influences the child’s susceptibility to chronic Non-Communicable Diseases (NCDs), including obesity, metabolic syndrome, diabetes, and allergies. Maternal dysbiosis is also recognized as a potential triggering factor in fetal neurodevelopment, being associated with neurodevelopmental disorders such as Autism Spectrum Disorder (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) [16,18].

c)

Therapeutic Potential: Given these intricate links, the maternal microbiota emerges as a potential therapeutic target. Identifying microbial risk factors highlights opportunities for developing microbiome-targeted predictive and therapeutic strategies to mitigate adverse outcomes [19,20].

3. Scope of the Review: Highlighting the Maternal–Infant Microbial Continuum:

This analysis focuses on the maternal–infant microbial continuum, encompassing the entire peripartum period and its long-lasting effects on the offspring. The scope involves evaluating microbial dynamics across multiple maternal niches the gut, vagina, oral cavity, and human milk oligosaccharides (HMOs), which contribute to vertical microbial transmission and shape the neonatal environment [1,2,3].

Key factors influencing this continuum and detailed within the research include:

a)

Vertical Transmission and Delivery Mode: The microbial composition acquired by the infant is heavily influenced by the mode of delivery. Vaginal birth facilitates colonization by beneficial maternal microbes, while Caesarean section (CS) results in aberrant colonization, which is associated with increased disease risk [24].

b)

Maternal Exposome and Interventions: The maternal microbiota is shaped by external influences, referred to as the exposome. This includes diet (e.g., Mediterranean diet, high-fat diets), antibiotic use, and maternal stress. Interventions such as probiotic supplementation or vaginal seeding are explored for their potential to modulate microbial health in both mother and infant [7,23].

c)

Postnatal Factors: The review addresses the continuous role of postnatal factors, such as breastfeeding, which supplies beneficial bacteria and Human Milk Oligosaccharides (HMOs) essential for infant immune and microbial development [41,42].

4. Pregnancy Microbiome Changes

The maternal microbiome, encompassing multiple anatomical sites, undergoes significant compositional and functional changes during pregnancy, which are vital physiological adaptations designed to support fetal development and prepare the mother for birth and lactation. These changes are largely driven by hormonal and metabolic shifts [3,7].

4.1. Gut Microbiome Shifts: Metabolic Adaptations to Pregnancy:

The gut microbiota undergoes profound remodeling during pregnancy, a process best understood as a metabolic adaptation to meet the heightened energetic and nutritional demands of the growing fetus [3]. Compositionally, this is characterized by a shift in the relative abundance of key phyla. There is typically an increase in Firmicutes and a decrease in Bacteroidetes [1,3], a ratio associated with enhanced energy harvest and storage. Concurrently, rises in Proteobacteria and Actinobacteria towards the third trimester may contribute to a preparatory, low-grade pro-inflammatory state, potentially bolstering defense against pathogens. At the genus level, these shifts often involve an increase in saccharolytic bacteria like Lactobacillus and Bifidobacterium, alongside a decrease in beneficial, anti-inflammatory genera such as Faecalibacterium, a key producer of short-chain fatty acids (SCFAs). Functionally, these taxonomic changes are driven by and interact with pregnancy hormones like progesterone, which alters gut motility and permeability. The overall reduction in microbial diversity (alpha diversity) in late gestation, coupled with diminished SCFA production, has been mechanistically linked to the pathophysiology of gestational diabetes mellitus (GDM) [15,43], illustrating the critical link between microbial community structure, metabolic function, and pregnancy health (Figure 2).

4.2. Vaginal Microbiome: Dominance of Lactobacillus and Protective Role:

The vaginal microbiota (VMB) undergoes substantial, protective transformations during gestation, playing a critical role in preventing infection and forming a protective barrier for the fetus.

4.2.1. Dominance and Mechanism:

• Lactobacillus Dominance: The most notable change in the VMB is the shift toward a less diverse, more stable state dominated by Lactobacillus species. In nonpregnant women, the VMB is typically classified into Community State Types (CSTs), four of which are dominated by Lactobacillus spp. Such as L. crispatus, L. gasseri, L. iners and L. jensenii [39,44].
• Hormonal Driver: Elevated estrogen and progesterone levels during pregnancy stimulate vaginal epithelial maturation, leading to the accumulation of glycogen. Lactobacillus utilizes this glycogen via fermentation to produce lactic acid [45].
• Protective Function: Lactic acid maintains a low vaginal pH (typically acidic), which is essential for preserving reproductive health. This acidic environment reduces microbial diversity and inhibits the growth of pathogenic microorganisms. Furthermore, Lactobacillus spp. stimulate the production of anti-inflammatory cytokines, enhancing maternal–fetal protection [46,47].

4.2.2. Species Differences and Adverse Outcomes:

• Protective Species: The species Lactobacillus crispatus is highly represented during pregnancy and its dominance is associated with low infection risk and favourable pregnancy outcomes. A higher relative abundance of Lactobacillus in early pregnancy is associated with a greater likelihood of the pregnancy continuing beyond 38 weeks [48].
• Dysbiosis and Pathogenesis: When dysbiosis occurs, Lactobacillus abundance decreases, and the environment is taken over by facultative anaerobes such as Gardnerella vaginalis, Prevotella, Megasphaera, Sneathia, and Atopobium vaginae. This dysbiosis, known as Bacterial Vaginosis (BV), is linked to an increase in local innate immune system activation and inflammation [49], which may contribute to complications such as Preterm Birth (PTB) (Figure 2) and Premature Pre-Labor Rupture of Membranes (PPROM) [13,50].
• Lactobacillus iners Controversy: The role of L. iners is controversial; while it is a Lactobacillus species, it is sometimes associated with vaginal dysbiosis, offers less protection against adverse pregnancy sequelae, and may be a marker of a transitional microbial state [39,51].

4.3. Oral Microbiome Changes and Links to Adverse Pregnancy Outcomes

The oral microbiome is also sensitive to the hormonal milieu of pregnancy. Elevated estrogen and progesterone levels can increase gingival vascular permeability and alter the subgingival environment, promoting a shift towards a more pathogenic community [22,52]. This dysbiosis is characterized by an increased abundance of periodontopathogens such as Porphyromonas gingivalis, Aggregatibacter actinomycetemcomitans, and Prevotella intermedia.

These changes can exacerbate gingival inflammation and contribute to pregnancy-associated gingivitis and periodontitis. Critically, this local oral dysbiosis has systemic implications. Via hematogenous dissemination (the oral–placental axis), oral bacteria like Fusobacterium nucleatum can translocate to the placenta, triggering inflammatory cascades that may lead to adverse outcomes including preterm birth, preterm pre-labor rupture of membranes (PPROM), and low birth weight [38,52]. Furthermore, maternal psychological stress has been associated with distinct oral microbial profiles, suggesting a complex interplay between neuroendocrine, immune, and microbial factors in determining pregnancy health [53].

5. Transmission Pathways:

The initial colonization of the neonate’s microbial communities is a critical event that occurs through various maternal and environmental pathways during the perinatal period. This process, referred to as vertical transmission or microbial seeding, sets the foundation for the infant’s long-term immune and metabolic health. Key maternal-to-infant microbial transmission pathways are summarized in Table 1.

The transfer of microbial communities from the mother to the infant, known as vertical transmission or microbial seeding is a pivotal event that establishes the neonate’s initial microbiome, influencing immune programming and long-term health outcomes. This process occurs through multiple routes: prenatal, during delivery, and postnatally [2,6,54].

5.1. The Placental Microbiome: Resolving the Controversy

The question of whether a resident microbial community exists in the healthy placenta and thus whether in utero microbial colonization occurs, remains a pivotal topic in reproductive microbiome research. The traditional “sterile womb” paradigm was challenged by early metagenomic sequencing studies reporting bacterial DNA in placental tissue, amniotic fluid, and meconium [11,55]. However, subsequent rigorous, contamination-controlled studies have critically reevaluated these claims [9,38,56].

The primary challenge lies in the extremely low microbial biomass of placental samples, making detected signals highly susceptible to contamination from DNA extraction kits, laboratory environments, or delivery. When stringent controls are applied, including sequencing of technical blanks and delivery room air, many putative placental microbial signatures become indistinguishable from background noise. Current high-quality evidence suggests the healthy term placenta is largely sterile, with previous detections likely representing either low-level, non-pathogenic microbial translocation during pregnancy or technical artifacts [9,11,38].

Nevertheless, pathogenic translocation to the placenta is a well-established route for infection-driven adverse outcomes. For example, hematogenous spread of oral pathogens like Fusobacterium nucleatum can cause placental inflammation leading to preterm birth [38,52]. Therefore, while a stable, replicating placental microbiome contributing to fetal development appears unlikely, the placenta remains susceptible to dysbiosis in distant maternal sites via systemic circulation. Potential routes for such translocation are illustrated in Figure 1.

Future research must employ meticulous, contamination-aware protocols to distinguish true biological signal from artifact in this critical, low-biomass niche [11].

5.2. Vaginal Delivery vs. Caesarean Section: Microbial Seeding of the Neonate:

The mode of delivery is a major and persistent factor influencing the initial composition and diversity of the infant gut microbiota, with both short- and long-term implications for health.

5.2.1. Vaginal Delivery (VB):

• Microbial Exposure: Neonates born vaginally are exposed to the mother’s vaginal, fecal, and perineal microbiota as they pass through the birth canal. This process is considered the physiological colonization standard [1,57].
• Colonization Pattern: VB facilitates colonization by beneficial species such as Bifidobacterium, Lactobacillus, Escherichia coli, and Bacteroides. These Commensal bacteria play pivotal roles in fermenting Human Milk Oligosaccharides (HMOs) [42].
• Immune Impact: VB induces immediate activation of intestinal epithelial cells by bacterial lipopolysaccharides (LPS), which promotes immune tolerance, intestinal barrier maturation, and a more balanced Th1/Th2 immune response. Vaginally delivered infants have been shown to have a healthier and more diverse bifidogenic microbiota compared to Caesarean Section (CS)-delivered infants [58].
• Source Contribution: While it is often assumed that vaginal flora is the critical source, increasing evidence suggests that taxa from the mother’s rectum and fecally-contaminated perineal surface are a more important source of engrafting intestinal bacteria during vaginal birth than the vagina itself. In vaginally-born neonates, the gut colonization is predominantly contributed by the maternal rectal microbiota rather than the vaginal microbiota at one month of age. Shared ASVs (Amplicon Sequence Variants) between the neonate and maternal rectal swab often come from the genera Bacteroides and Bifidobacterium [2].

5.2.2. Caesarean Section (CS):

• Microbial Exposure: CS delivery bypasses exposure to the maternal vaginal and fecal microbiota, leading to the acquisition of a less diverse intestinal microbiome. Infants born via CS acquire microbiota dominated by environmental bacteria and nosocomial pathogens found in the hospital, such as Staphylococcus and Corynebacterium [24].
• Colonization Pattern: CS-born infants show delayed and altered colonization patterns. They have reduced colonization by Bifidobacterium and Bacteroides and are often dominated by opportunistic pathogenic species like Enterococcus, Staphylococcus, Streptococcus, Klebsiella, and Clostridium perfringens. The gut microbiota profile of CS-born neonates may be more similar to their mother’s skin communities [24,59].
• Immune Consequences: This altered colonization is associated with a reduced innate immune response, fewer regulatory T cells (Tregs), and a less diverse and more unstable microbiota. The limited microbial diversity in CS-born infants delays immune maturation.
• Persistent Effects: These changes in gut microbiota have been observed to persist for up to 1–2 years after birth. CS delivery is strongly associated with a significantly lower microbial diversity (Chao1 alpha diversity) on the neonatal skin compared to VB neonates. CS-delivered neonates also had a significantly higher relative abundance of the potential opportunistic pathogen Ureaplasma in their oral microbiome [60].

5.2.3. Intervention Strategies:

To counteract the dysbiosis associated with CS, interventions have been explored:

• Vaginal Seeding: This involves applying maternal vaginal fluids to the newborn immediately after CS. While this technique has shown promise in some small, controlled studies, sometimes improving gut microbiota, metabolome, and neurodevelopmental scores in actively treated infants, results have been inconsistent. Some research found that oral administration of maternal vaginal organisms did not modify the infants’ intestinal microbiome. Furthermore, vaginal seeding raises ethical and safety concerns regarding the potential transmission of pathogenic microorganisms [23].
• Fecal Microbiota Transplantation (FMT): Sincematter is a source of initial colonization in VB infants, FMT using the mother as a donor has been trailed for CS-born infants, showing that this approach can rapidly restore normal gut microbial development [59].

5.3. The Placental Microbiome: Evaluating the Evidence for in Utero Colonization

The question of whether a resident microbial community exists in the healthy placenta and thus whether in utero microbial colonization occurs remains one of the most contentious topics in reproductive microbiome research. The traditional “sterile womb” paradigm was challenged by early metagenomic sequencing studies reporting bacterial DNA in placental tissue, amniotic fluid, and meconium. However, subsequent rigorous, contamination-controlled studies have critically reevaluated these claims [11].

The primary challenge lies in the extremely low microbial biomass of placental samples, making detected signals highly susceptible to contamination from DNA extraction kits, laboratory environments, or delivery. When stringent controls are applied including sequencing of technical blanks and delivery room air many putative placental microbial signatures become indistinguishable from background noise [9,11]. High-quality evidence now suggests the healthy term placenta is largely sterile, with previous detections likely representing either low-level, non-pathogenic microbial translocation during pregnancy or technical artifacts [8,11].

Nevertheless, pathogenic translocation to the placenta is a well-established route for infection-driven adverse outcomes. For example, hematogenous spread of oral pathogens like Fusobacterium nucleatum can cause placental inflammation leading to preterm birth [38]. Therefore, while a stable, replicating placental microbiome contributing to fetal development appears unlikely, the placenta remains susceptible to dysbiosis in distant maternal sites via systemic circulation (Table 2).

Future research must employ meticulous, contamination-aware protocols to distinguish true biological signal from artifact in this critical, low-biomass niche.

5.4. Human Milk Oligosaccharides (HMOs) as a Microbial and Immunological Bridge:

Human milk is a biologically active fluid that serves as a fundamental and safe strategy for shaping the neonatal microbiota and immune system, acting as a direct “bridge” between the maternal and infant worlds [68].

5.4.1. Microbial Source

• Diverse Community: Breast milk contains a diverse microbiota, with between 590 and 600 bacterial genera identified. The main bacterial groups include Firmicutes (Staphylococcus, Streptococcus, Lactobacillus, Bifidobacterium), Proteobacteria (Escherichia, Enterobacter), and Actinobacteria [68].
• Origin Hypotheses: Four main hypotheses regarding the origin of breast milk bacteria exist [68]:
  • Enteromammary Route: Bacteria from the maternal gut are transported by immune cells (macrophages, dendritic cells) to the mammary gland, supporting the presence of anaerobic bacteria like Bifidobacterium.
  • Infant’s Oral Cavity: Retrograde flow due to the sucking motion carries bacteria from the infant’s mouth into the breast tissue, supported by the abundance of Streptococcus in both infant saliva and breast milk.
  • Maternal Skin: Contamination from the skin, supported by the presence of skin-associated bacteria (Staphylococcus, Corynebacterium).
  • Intrinsic Mammary Microbiota: The mammary gland may host its own stable microbiota [68].
• Impact on Infant Gut: It is estimated that between 10% and 27% of the infant’s gut microbiota originates from breast milk. Breastfed infants tend to have microbiota dominated by Bifidobacterium, promoting immune system maturation and potentially reducing the risk of chronic diseases like allergies and obesity later in life [68].

5.4.2. Immunological and Prebiotic Components

• Human Milk Oligosaccharides (HMOs): human milk contains essential oligosaccharides (HMOs) that function as selective substrates (prebiotics) for beneficial bacteria, particularly Bifidobacterium species [68,69,70].
  • HMOs promote the growth of specific bacteria (B. longum subsp. Infantis, B. bifidum) which produce Short-Chain Fatty Acids (SCFAs) like acetate and lactate, lowering intestinal pH and inhibiting pathogens [70].
  • HMOs also act as decoy receptors for pathogens (e.g., Campylobacter jejuni, E. coli), preventing their adhesion to the gut epithelium, and modulate the expression of inflammatory genes, supporting immune defense [70].
• Maternal Factors: The composition of the human milk oligosaccharides (HMOs) microbiota is influenced by maternal factors, including gestational age (higher Bifidobacterium in term mothers), mode of delivery (VB linked to higher Bifidobacterium, Lactobacillus), maternal BMI, and antibiotic exposure (which reduces Bifidobacterium and Lactobacillus) [70].

5.5. Skin and Environmental Exposure Shaping Early Colonization:

Beyond the primary maternal sources, the environment and maternal skin contribute significantly to the neonatal microbial profile, particularly immediately after birth [33].

• Skin-to-Skin Contact (Kangaroo Care): This practice promotes maternal and neonatal bonding and exerts a beneficial effect on the development of the gut microbiota. Kangaroo care increases vertical microbial transmission from mother to infant [71]. Skin-to-skin contact has been shown to improve the oral microbial composition, promoting the growth of Streptococcus and reducing pathogenic bacteria like E. coli.
• Environmental Exposure (Hospital/NICU): In CS-born infants, the initial microbial environment is often acquired from the general environment of the hospital [58]. In Neonatal Intensive Care Units (NICUs), the neonatal unit environment, including equipment and surfaces, influences the acquisition of bacteria by premature infants. Each NICU room has been shown to possess a unique microbial signature [58].
• Non-Maternal Human Contact: Horizontal transmission of microbial species from family members (e.g., fathers and siblings) is another significant source of colonization (Table 1), influencing the neonatal microbiome after the initial period. One study suggested that fathers introduced most new shared strains [54].
• Home/Farm Exposure: Direct exposure to microbes via hygienic factors or unique environments (e.g., soil, animals) contributes to the evolution of the child’s microbiome and long-term health. Children raised on farms, exposed to animals, showed reduced frequency of later allergies and inflammatory bowel disease [72].

6. Infant Microbiome Trajectories

The first 1,000 days of life after birth from conception through the first two years after birth, constitute a critical window of opportunity during which the infant’s gut microbiome undergoes rapid development, influencing immune programming, metabolic pathways, and long-term health outcomes. The establishment of the infant’s microbial ecosystem is a dynamic, multi-stage process shaped by maternal microbial communities and various perinatal and environmental factors [25,26].

6.1. Dynamics During the First 1,000 Days of Life:

The colonization of the infant gut is a pivotal process that influences health and development (Table 2). The microbial composition generally tends to mature into an adult-like microbiota after the first two years’ post-partum, or possibly up to five years or even longer [73].

6.1.1. Key Dynamics and Shifts:

• Initial Colonization (Birth to Day 4): The initial microbial exposure occurs during birth, and the neonatal gut microbiome undergoes rapid changes immediately after birth [58].
  • Alpha diversity (richness) may initially decline before increasing again. In one cohort, alpha diversity was significantly lower on Day 4 compared to Day 0 and Day 1.
  • The bacterial composition changes significantly shortly after birth (Days 1, 4, and 1Month). Taxa such as Staphylococcus, Rothia, Streptococcus, and Haemophilus were observed to be higher around Day 4 than Day 1 [58].
• Maturation Phase (Post-Weaning): The introduction of solid foods (weaning) at approximately four to six months of age alters the gut microbiota, moving it toward a more mature profile [74].
  • This change is driven by exposure to dietary carbohydrates and glycans.
  • The microbiome complexity expands, typically featuring a rise in strains from the phyla Firmicutes and Bacteroidetes, along with increases in Atopobium, Clostridium, Akkermansia, Lachnospiraceae, and Ruminococcus [74,75].
  • The intestinal microbiome development in the first 2–3 years of life is considered the most critical phase [25].

6.1.2. Intervention Opportunities:

Early probiotic supplementation has emerged as a promising strategy in preterm infants. Meta-analyses demonstrate that prophylactic probiotics (particularly Bifidobacterium and Lactobacillus combinations) reduce NEC risk by 30-50% (RR 0.5-0.7) and all-cause mortality [Reference]. However, optimal strains, dosing, and timing remain to be established, and safety monitoring is critical given the vulnerability of this population.

6.2. Factors Shaping Colonization: Feeding Mode, Antibiotic Exposure, Maternal Health

The establishment of the infant microbiome is shaped by several endogenous and exogenous factors, collectively referred to as the exposome [33,72].

6.2.1. Mode of Delivery

The delivery mode exerts a major and persistent effect on the composition and diversity of the infant gut microbiota [69].

• Vaginal Birth (VB): Vaginally born neonates are exposed to maternal vaginal and rectal microbiota, facilitating colonization by beneficial, bifidogenic microbiota [69].
  • VB infants show higher levels of Bifidobacterium, Lactobacillus, and Bacteroides.
  • This physiological process promotes early immune tolerance and barrier maturation.
  • In vaginally delivered neonates, the gut microbiome becomes significantly more similar to the maternal gut microbiome over time compared to the vaginal microbiome [69].
• Caesarean Section (CS): CS delivery results in an aberrant intestinal microbial colonization and a less diverse intestinal microbiome [69].
  • CS infants acquire microbiota dominated by environmental bacteria and opportunistic pathogens from the hospital setting, such as Staphylococcus, Enterococcus, Streptococcus, Klebsiella, and Clostridium perfringens [58].
  • They show reduced colonization by Bifidobacterium and Bacteroides [58].
  • This altered colonization pattern, which may persist for up to 1–2 years after birth, is linked to long-term health problems such as allergies, asthma, obesity, and Type 1 diabetes [72].

6.2.2 Feeding Mode (Breastfeeding vs. Formula)

Breast milk is a fundamental and safe strategy for modulating the neonatal microbiota.

• Breastfeeding: Breastfed infants typically exhibit a healthier, diverse microbial composition [68].
  • They have microbiota dominated by Bifidobacterium, Lactobacillus, and Bacteroides.
  • Breast milk contains essential Human Milk Oligosaccharides (HMOs), which act as selective substrates (prebiotics) for beneficial bacteria, particularly Bifidobacterium species [70].
  • Breastfed infants are 30–40% less likely to develop allergies compared with those fed formula [72].
• Formula Feeding: Formula-fed infants develop a more diverse, adult-like microbiota earlier [69].
  • They often show a higher prevalence of potentially pathogenic taxa, including Clostridium, Enterobacteriaceae, Streptococcus, and C. difficile, and low levels of Bifidobacterium [69].
  • Supplementation of infant formulas with prebiotics like Galactooligosaccharides (GOS) and Fructooligosaccharides (FOS) can improve the infant gut microbiome to be more like breastfed infants [76].

6.2.3. Antibiotic Exposure

Antibiotic use during pregnancy or the perinatal period is a significant factor linked to dysbiosis that can interfere with vertical transmission [77].

• Maternal/Perinatal Exposure: Antibiotics given during pregnancy significantly alter the gut and vaginal microbiome of the mother and, more importantly, lead to a sustained drop in abundance of Bifidobacterium and Bacteroidetes in the newborn [78].
• Neonatal Exposure: Antibiotic administration in neonates induces dysbiosis, characterized by reduced bacterial diversity (reduced Shannon index/alpha diversity), decreased or absent Bifidobacteria species, and a higher abundance of pathogenic bacteria like Enterobacteriaceae [78].
  • The damage to the intestinal microbiome from perinatal antibiotics can persist for at least one year of age, a critical duration for immune system development, and can result in future allergic and metabolic consequences.
  • Each additional day of antibiotic exposure in newborn infants is associated with a reduction in obligate anaerobic bacterial load in stools [78].
• Increased Risk of Allergies: Maternal antibiotic use during pregnancy was strongly associated with an increased risk of allergies in the offspring (OR = 1.8–2.6) [12].

6.2.4 Other Maternal and Environmental Factors

• Maternal Stress: Maternal psychosocial stress during pregnancy is associated with changes in gut microbiota diversity that may undermine neonatal immunity (). This stress has been associated with an increased chance of immune problems in babies (OR = 1.9–2.4) [12].
• Maternal Obesity: Maternal obesity is associated with alterations in infant gut microbiota and metabolism. Neonates born to obese mothers showed a higher relative abundance of opportunistic genera like Peptoniphilus on their skin and Bacteroides and Ruminococcus in their oral cavities, potentially increasing infection risk [79].
• Environmental Exposure: Factors like skin-to-skin contact (which promotes vertical microbial transmission) and exposure to the Neonatal Intensive Care Unit (NICU) environment (which can introduce unique microbial signatures from equipment and surfaces) significantly influence colonization, especially in premature infants [58,71].

6.2.5 Microbiome–Immune System Interactions in Tolerance and Training:

The infant microbiome trajectory is intrinsically linked to the programming and development of the gut immune system, setting the stage for long-term health or disease.

• Immune Imprinting and Tolerance: The early relationship between commensal microbes and the developing immune system is called immune imprinting, which fine-tunes immune cells for appropriate responses and tolerance to harmless environmental antigens [27,80].
  • Exposure to microbes early in life is essential to the development of a complete functioning immune system, but this exposure must occur at an early age [27].
  • Vaginal birth promotes immune tolerance and a balanced Th1/Th2 immune response, fostering a richer and more functional microbiota [69].
• Protection Against Pathogens: The indigenous microbiota provides colonization resistance against enteric pathogens like Cryptosporidium parvum. The absence of a healthy diverse microbiota (dysbiosis) can lead to alteration of the immune system and the development of atopic disorders and food allergies [27].
• Metabolite Mediation: Maternal gut microbiota-derived metabolites, such as Short-Chain Fatty Acids (SCFAs), play an essential role in shaping the fetal immune system and neurological development. A reduction in SCFA availability due to dysbiosis is linked to enhanced immune reactivity and chronic inflammation [27].
• Vaccine Response: A healthy microbiome is important for the immune response to pathogens and to vaccines, which results in protective immunity. Higher abundance of Bifidobacterium infantis in the early months is associated with stronger CD4+ T-cell responses and more robust immune memory after vaccination [27].
• Interventions to Support Immunity:
  • Probiotic supplementation during pregnancy or early life is associated with a 20–40% reduction in allergy risk (RR = 0.6–0.8), suggesting that modulating the maternal gut microbiota confers long-term benefits to the infant immune system [20].
  • Breastfeeding provides immune components (e.g., IgA) and beneficial bacteria that help build the baby’s immune system and prevent allergic reactions caused by inflammation [68].

7. Microbial Dysbiosis and Disease Links in Mother & Infabts

Microbial dysbiosis, defined as a qualitative and/or quantitative alteration of microbial communities, is closely linked to several pathological conditions in both the pregnant mother and the developing offspring [4].

7.1. Maternal Dysbiosis: Preeclampsia, Gestational Diabetes, Preterm Birth:

Dysbiosis across maternal niches, including the gut (Table 3), vagina, oral cavity, and potentially the placenta is associated with adverse pregnancy outcomes and obstetric diseases [4].

7.2. Preeclampsia (PE)

PE is a serious complication characterized by hypertension and often proteinuria, linked to oxidative stress, systemic inflammation, and abnormal placental remodeling.

• Gut Dysbiosis and PE: Women with PE show gut dysbiosis, characterized by a reduction in beneficial bacteria, such as Faecalibacterium and Akkermansia, and an increase in opportunistic pathogens, such as Fusobacterium and Veillonella [51].
  • This dysbiosis reduces the production of Short-Chain Fatty Acids (SCFAs), which typically suppress inflammation and support gut barrier integrity.
  • Dysbiosis also contributes to elevated circulating levels of lipopolysaccharides (LPS) from Gram-negative bacteria and Trimethylamine-N-oxide (TMAO). TMAO exacerbates endothelial dysfunction and intensifies gestational hypertension [28].
• Vaginal and Oral Links to PE: Vaginal dysbiosis (loss of Lactobacillus) promotes local inflammation, increasing the risk of placental dysfunction [49]. Furthermore, oral dysbiosis and associated periodontal disease have been plausibly linked to PE pathogenesis, potentially through the translocation of oral bacteria like Prevotella, Porphyromonas, and Dialister to the placenta [52,53].

7.3. Gestational Diabetes Mellitus (GDM) and Insulin Resistance

Gut dysbiosis during pregnancy mediates the risk of developing GDM.

• Gut Dysbiosis in GDM: Women with GDM exhibit a significant reduction in microbial diversity compared to healthy women, displaying a pro-inflammatory bacterial profile [15]. There is often an increase in taxa associated with inflammation and insulin resistance, such as Firmicutes and Proteobacteria, and a decrease in beneficial taxa like Bacteroidetes.
  • A reduction in SCFA-producing bacteria may heighten GDM risk by promoting proinflammatory bacteria that interfere with glucose metabolism [43].
  • GDM has been predicted from as early as Week 12 of pregnancy using clinical and microbiome characteristics [15].

7.4. Preterm Birth (PTB)

Dysbiosis is a key factor in PTB and Preterm Prelabor Rupture of Membranes (PPROM).

• Vaginal Dysbiosis and PTB: Alterations in the vaginal microbiota (VMB), particularly a loss of Lactobacillus species and an increase in opportunistic pathogens (e.g., Gardnerella vaginalis, Prevotella, Sneathia, and Atopobium vaginae), are strongly associated with PTB and PPROM [13].

• a. A significant decrease in the abundance of Lactobacillus crispatus in the VMB during the last trimester may be linked to preterm delivery [13]. Conversely, a higher relative abundance of Lactobacillus in early pregnancy is associated with a greater likelihood of pregnancy continuation beyond 38 weeks [48].

• Gut Dysbiosis and PTB: Maternal gut dysbiosis in early pregnancy can predict PTB. Notably, the genus Clostridium innocuum was identified as a predictive marker for PTB across independent cohorts. C. innocuum is known to degrade 17β-estradiol in vitro and in vivo, suggesting that early-pregnancy microbial dysbiosis may initiate hormonal dysbiosiss that trigger preterm labor [39].

7.5. Infant Dysbiosis as Disorders:

The initial microbial colonization (or lack thereof, such as reduced diversity via Cesarean section or antibiotics) places infants on a long term path that may lead to chronic non-communicable diseases (NCDs) [4].

8. Mechanistic Pathways Linking Dysbiosis to Pathology

The link between dysbiosis and disease relies on complex molecular and immunological mechanisms, which can be organized into four core pathways (Table 4):

• Hormonal Metabolism: Certain pathogenic bacteria can directly interfere with host endocrinology. For example, Clostridium innocuum degrades 17β-estradiol, potentially disrupting the hormonal balance necessary for maintaining pregnancy and increasing PTB risk [4].
• Immunomodulation & Barrier Integrity: Commensal bacteria and their metabolites are essential for immune education and barrier function. Short-chain fatty acids (SCFAs) promote regulatory T cell differentiation and anti-inflammatory cytokine production [27]. Conversely, reduced SCFA availability impairs gut barrier integrity, leading to inflammation. Vaginal Lactobacillus spp. maintain a low pH and produce lactic acid, protecting against ascending infections [13].
• Microbial Metabolite Signaling: The gut microbiota modulates host metabolism via bile acid transformation. Dysbiosis-induced changes in bile acid pools can affect host metabolism and pathogen susceptibility. For instance, Veillonella parvula suppresses the bile acid transporter ASBT, leading to bile acid accumulation and C. difficile spore germination [81].
• Pathogen Exclusion & Competition: A healthy, diverse microbiota provides colonization resistance against pathogens. Bifidobacterium species, promoted by human milk oligosaccharides (HMOs), outcompete pathogens for resources and produce inhibitory metabolites [70].

8.1. Clinical Implications and Its Gaps

The evidence for the maternal-infant microbial continuum has several immediate and future implications for clinical practice:

8.1.1. Perspectives for Stakeholders for Clinicians and Healthcare Providers:

• Screening Potential: Maternal microbiome profiling may emerge as a novel biomarker for pregnancy risk stratification (e.g., C. innocuum for PTB risk) [4,13].
• Preventive Counseling: Emphasize modifiable factors: Mediterranean-style diets, judicious antibiotic use, stress reduction techniques, and smoking cessation [10,29].
• Birth Planning: Discuss the microbial benefits of vaginal delivery when medically appropriate, while respecting patient autonomy and circumstances [24,26].
• Postnatal Support: Actively promote and support exclusive breastfeeding for at least 6 months, given its proven benefits for microbiome and immune development.

8.1.2. For Researchers:

• Priority Studies: Large, longitudinal RCTs with standardized protocols are urgently needed to establish causality and intervention efficacy [10].
• Personalization: Move beyond “one-size-fits-all” approaches toward precision microbiome medicine based on individual microbial and genetic profiles [19,32].
• Mechanistic Depth: Focus on functional metagenomics and metabolomics to move from correlation to mechanistic understanding [10].

8.1.3. For Policy Makers and Public Health:

• Guideline Development: Support the development of evidence-based guidelines for probiotic use in pregnancy and infancy [10].
• Education Initiatives: Fund public health campaigns about the importance of early-life microbiome health [32].
• Research Investment: Prioritize funding for longitudinal mother-infant cohort studies across diverse populations [10].

8.2. Gaps and Challenges in Translating Interventions to Clinical Practice:

Translating promising findings into routine clinical practice faces several significant hurdles:

• Causality vs. Association: Most studies are observational, limiting the ability to establish definitive causal relationships between dysbiosis and pregnancy-related diseases [32].
• Heterogeneity and Standardization: Current studies show significant heterogeneity in design, methodologies, sample size, and outcomes, making it difficult to draw definitive conclusions. There is a critical need for standardization in sampling, analysis, and data reporting methodologies [32].
• Intervention Optimization: Key gaps remain in identifying the optimal probiotic strains, dosages, and timing for interventions. Probiotic efficacy is highly variable, and the unregulated market poses safety concerns [82].
• Underrepresentation and Generalizability: The underrepresentation of diverse populations in current research limits the generalizability of findings regarding microbial composition and disease risk [11].
• Lack of Longitudinal RCTs: There is a lack of large-scale, well-designed Randomized Controlled Trials (RCTs) to validate the safety and efficacy of microbiota-based interventions across diverse populations.
• Complex Interactions: Future research must address the intricate interplay between microbiota, genetics, psychosocial stress, immune profile, and placental epigenetics [30].

9. Interventions and Preventive Strategies

Microbiota-targeted interventions are increasingly explored as promising, non-invasive approaches to prevent or mitigate adverse maternal and neonatal health outcomes [19].

9.1. Maternal Diet and Nutritional Interventions

Diet is a primary, modifiable factor that profoundly influences maternal and neonatal microbial composition.

• Mediterranean Dietary Pattern (MD): The MD, characterized by high consumption of fiber, fruits, vegetables, and unsaturated fats, is associated with greater bacterial diversity and a protective effect against disease risk [7].
  • An MD-based counselling intervention during pregnancy significantly influenced the overall structure of the maternal gut microbiota.
  • MD adherence was associated with an increase in gut microbial diversity and the enrichment of beneficial genera like Blautia, Faecalibacterium, and Christensenella[7].
  • MD promotes SCFA production, enhances intestinal barrier integrity, and reduces systemic inflammation, mitigating the release of proinflammatory cytokines [29].
• Fiber and PUFAs: Diets rich in dietary fiber and polyunsaturated fatty acids (PUFAs) support the growth of beneficial species like Lactobacillus and Bifidobacterium, improve vascular function, and are associated with healthier neonatal microbiota and improved infant developmental outcomes [7,65].
• Stress Reduction (SR): A structured Stress Reduction program during pregnancy showed a significant link with the maternal gut microbiota composition, modulating the relative abundance of some key health-associated genera. High maternal stress was associated with lower relative abundance of SCFA producers like Faecalibacterium [53].

9.2. Probiotics, Prebiotics, and Synbiotics in Pregnancy and Infancy:

Microbiota modulation using biotics is a key therapeutic strategy.

• Probiotics in Pregnancy: Probiotic supplementation during pregnancy may be associated with a reduced risk of GDM and other complications.
  • Probiotic administration has been shown to improve glucose and lipid metabolism in pregnant women and may reduce allergic symptoms in newborns [20].
  • Probiotics can improve diversity of the intestinal or vaginal microbiome and insulin sensitivity, and reduce inflammation, as seen in studies using multi-strain products [20].
  • Some studies, however, report mixed results regarding their impact on GDM prevention in overweight/obese women [31].
• Prebiotics in Pregnancy and Infancy: Prebiotics, such as fructans (FOS and inulin) and Galactooligosaccharides (GOS), are soluble fibers that selectively promote the growth of beneficial bacteria, such as those that produce SCFAs. Prebiotic supplementation during pregnancy modified the gut microbiota and increased metabolites in amniotic fluid, driving a tolerogenic environment in utero [76].
• Vaginal Modulation: Therapeutic strategies utilizing oral or local probiotics (e.g., Lactobacillus crispatus and L. rhamnosus) are being explored to modulate vaginal microbiota, particularly when dysbiosis is present (Table 3). This dual approach may reduce proinflammatory markers and restore microbial balance, diminishing the risk of reinfection and inflammatory flare-ups [40].

9.3. Microbiome Restoration Strategies:

• Vaginal Seeding: This involves exposing Caesarean Section (CS)-born neonates to maternal vaginal microbiota to partially restore physiological colonization [23]. While studies are small and results mixed, some have shown that vaginal microbiota transfer may improve the neurodevelopmental scores and microbiome composition in CS-born infants [67]. However, oral administration of maternal vaginal organisms has also been shown not to modify the infants’ intestinal microbiome in some trials [23].
• Fecal Microbiota Transplantation (FMT): FMT involves harnessing fecal-derived microbiota to reverse dysbiosis. FMT has been successfully used to treat recurrent C. difficile infection (CDI) in pregnant women, resulting in engraftment of the donor microbiota which was later transferred to the infant during vaginal delivery. This technique is typically limited to research centers as part of IRB-approved clinical trials [58].
• Exclusive Breastfeeding: Breast milk remains the gold standard for promoting healthy gut microbiota development. It contains prebiotics (HMOs) and live probiotic bacteria that shape the infant gut microbiota and support immune development. This influence may reduce the risk of chronic diseases such as allergies, obesity, and diabetes later in life [68].
• Donor Milk (DM) Modulation: DM, often pasteurized, loses some probiotic benefits. Future strategies should consider personalization of DM by adding components of mother’s own milk or specific human milk components to improve its quality [68]. Research indicates that the use of prebiotics, probiotic strains, and synbiotics in formula milk holds the potential to influence the establishment and development of infant gut microbiota when human milk is unavailable [66].

10. Conclusions

The evidence reviewed herein firmly establishes the maternal–infant microbial continuum as a critical determinant of perinatal and lifelong health [1,2]. From pregnancy through the first 1,000 days, maternal microbial communities guide fetal development, seed the infant microbiome, and instruct the nascent immune system. Dysbiosis within this continuum, whether in the maternal gut, vagina, or oral cavity is robustly linked to a cascade of adverse outcomes, from preterm birth and preeclampsia to childhood allergies and metabolic disorders [4,38,51].

This framework positions the maternal microbiome as a powerful, yet underutilized, target for prenatal primary prevention. Interventions such as dietary modulation, targeted probiotic supplementation, and support for breastfeeding aim to optimize microbial transmission and infant colonization [42]. However, moving from association to effective clinical application requires overcoming significant hurdles. Future research must prioritize large-scale, longitudinal, and diverse cohort studies employing standardized, contamination-aware methodologies. A multidisciplinary approach, integrating multi-omics data with clinical outcomes, is essential to unravel the precise mechanisms at play and to develop personalized microbiome-based strategies [10,20].

Ultimately, nurturing a healthy maternal microbiome may be one of the most fundamental and proactive investments we can make in the health of future generations. By bridging the gap between mechanistic insight and clinical practice, we can transform the promise of microbiome science into tangible improvements in maternal and child health outcomes [32].