Clinical meaning
The prescribing logic framework for pregnancy safety integrates embryologic timing, pharmacologic mechanism, and evidence-based risk classification to guide medication decisions during the reproductive period. The former FDA pregnancy letter categories (A, B, C, D, X), which were in use from 1979 to 2015, were fundamentally flawed because they implied a linear hierarchy of risk that oversimplified complex data — category C included 60% of all medications and provided no clinically actionable information. The 2015 Pregnancy and Lactation Labeling Rule (PLLR) replaced letters with narrative subsections providing human data summaries, animal data, pharmacokinetic considerations, and clinical implications. Prescribing logic requires understanding why specific drugs cause harm through their mechanisms: ACE inhibitors (lisinopril, enalapril) and ARBs (losartan, valsartan) are fetotoxic in the second and third trimesters because the fetal renin-angiotensin system is essential for nephron development and maintenance of fetal renal perfusion — inhibition causes renal tubular dysplasia, oligohydramnios (from decreased fetal urine output), pulmonary hypoplasia (from oligohydramnios), limb contractures, and skull ossification defects. Statins are contraindicated because cholesterol is essential for fetal cell membrane synthesis, steroid hormone production, and Hedgehog signaling...