How it works
Naegele's rule: estimated due date = first day of the last menstrual period + 280 days (40 weeks). Gestational age counts from that same date, not from conception (which usually happens about two weeks later). Trimester 1 runs to 13 weeks 6 days, trimester 2 to 27 weeks 6 days, and trimester 3 runs to the due date. Only about 1 in 20 babies arrives exactly on the due date, so treat it as a window, not an appointment.
Frequently asked questions
Why is the due date 40 weeks from my period, not from conception?
Because the period date is usually known more reliably than the conception date, and the convention counts pregnancy from it.
What if my cycles are irregular?
Then this estimate is less reliable, and an early ultrasound is the better dating method — ask a clinician.