Pregnancy Calculator

How it's calculated

Gestational age = date − first day of the last period (LMP)
Due date = LMP + 280 days (Naegele's rule), shifted by (cycle length − 28) days
From a conception date: LMP ≈ conception − 14 days; from a due date: LMP = due − 280 days

Pregnancy is counted from the first day of the last period, about two weeks before conception, so at conception you are already "two weeks pregnant". Only about one baby in twenty-five arrives on the due date itself; most come within two weeks either side. If an early scan gave a different date, use that due date here.