Conception Date vs. Pregnancy Dating: Understanding the Two-Week Difference
Obstetric dating counts from the first day of the last menstrual period (LMP), which precedes conception by ~2 weeks.
- Conception (fertilization): Sperm meets oocyte ~Day 14 of a 28-day cycle.
- Gestational age: LMP + 2 weeks = embryonic age + 2 weeks.
- Due date (EDD): LMP + 280 days (40 weeks) = conception + 266 days (38 weeks).
Thus, "4 weeks pregnant" by dates = embryo is 2 weeks post-conception. Early ultrasound (crown-rump length) provides the most accurate dating (±3–5 days) and may adjust the EDD.
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