Figure 4.

Inferring pregnancy start dates using the Estimated Start Date (ESD) algorithm. The 74 concepts determined to be gestational timing specific were split into 2 types: Gestational Range 3 months (GR3m) and Gestational Week (GW). GR3m indicated concepts that have a possible gestational timing span of >1 week but <3 months at the point they occur during pregnancy (eg, estriol can be tested between months 3.75–5.5). GW indicated concepts of type “Gestation period, X weeks,” denoted as point estimates since week-level was the smallest unit of precision for gestational timing in this study. Start dates or plausible start date ranges from these concepts were determined by extrapolating backwards using gestational timing information, and outliers were removed using the 1.5*IQR on each concept type and the overlap of intersecting start date ranges and point estimates (steps 1–5b). The most logical start dates were then assigned based on concept types present, and levels of precision of start date estimate were added (step 6).