Key Concepts
Introduction
Naegele's Rule: EDD = First day of LMP + 9 calendar months + 7 days Equivalent: EDD = LMP + 280 days (40 weeks) Example: - LMP = January 15, 2026 - Add 9 months: October 15, 2026 - Add 7 days: October 22, 2026 = EDD Assumptions: 28-day cycle with ovulation on day 14. Adjustments needed for: - Cycles >28 days (ovulates later โ EDD later): add extra days for each week of cycle length >28 - Cycles <28 days (ovulates earlier โ EDD earlier): subtract days Gestational age calculation: weeks + days since first day of LMP - Example: LMP May 1; today is July 24 โ 12 weeks 3 days First trimester ultrasound dating: - Crown-rump length (CRL) measurement at 7โ13+6 weeks is the most accurate dating method - CRL provides ยฑ5 days accuracy at 8โ10 weeks - Supersedes LMP if discrepancy: >5 days (before 9 weeks), >7 days (9โ15+6 weeks), >10 days (16โ21+6 weeks) Term vs preterm gestational age definitions: - Early term: 37 0/7 โ 38 6/7 weeks - Full term: 39 0/7 โ 40...