A Closed-Form Fertilisation-Age to PK-Sim Dummy-Age Mapping Enabling Daily and Weekly Pregnancy Physiology Vectors in Open Systems Pharmacology Physiologically Based Pharmacokinetic Modelling

In pregnancy physiologically based pharmacokinetic (PBPK) modelling within PK-Sim/Mobi, pregnancy stage is canonically parameterised on the fertilisation-age (FA) timescale, yet PK-Sim’s physiology database is indexed by chronological “Age”. The Open Systems Pharmacology (OSP) pregnancy framework therefore encodes FA on a dummy “Age” axis (30.00 years = FA 0 weeks; 30.75 years = FA 38 weeks) and generates pregnancy physiology vectors from FA 0 – 38 weeks discretised at 1-day intervals for database ingestion. Although the anchor points are publicly documented, an explicit closed-form transform and week-resolved lookup suitable for deterministic reproduction of daily physiology grids has not been routinely foregrounded in the literature or repository instructions. A unique affine mapping is derived, implied by the anchors, that provide forward and inverse equations in week- and day-space, quantifying rounding error in terms of FA-day misalignment, and supply a fertilization week (1-38) table for implementation-ready pregnancy virtual population creation in PK-Sim.