Pregnancies are dated from a moment that almost certainly was not the moment your baby was conceived. Here is why that math actually makes sense.
Calculators do the heavy lifting, but knowing what is going on underneath is what saves you money — or stress — later. Pregnancies are dated from a moment that almost certainly was not the moment your baby was conceived. Here is why that math actually makes sense.
Estimated Due Date = LMP + 280 days (Naegele's rule)
LMP (last menstrual period) is the only date most people remember. Ovulation is harder to pinpoint. The system trades a small dating error for huge real-world consistency.
T1: weeks 1–13 (organs form). T2: weeks 14–27 (growth, movement). T3: weeks 28–birth (final maturation, weight gain).
Early ultrasound (especially before week 13) is more accurate than LMP and will override it if the difference is over 7 days.
Past 42 weeks is post-term. Doctors typically discuss induction at 41+ weeks because placenta function declines.
What if my cycles are irregular? Use the EDD as a rough anchor and let early ultrasound confirm. Cycle length affects ovulation timing more than gestation length.
Can my due date change in late pregnancy? It can shift by a few days but rarely more. Late ultrasounds are less accurate at dating than early ones.
Whatever number you land on, sanity-check it against a second method or a quick estimate in your head. Try the Pregnancy Week Calculator linked above to put these numbers to work right now.