How is Due Date Calculated?
Most pregnancies last about 40 weeks (or 280 days) from the first day of your last menstrual period (LMP). However, conception usually happens about 2 weeks after your period starts.
The Naegele's Rule
Our calculator uses the standard medical formula known as Naegele's Rule: Add 7 days to the first day of your LMP, and then subtract 3 months. This gives the estimated date of delivery (EDD).
Why "Estimated"?
Only about 4% of babies are born on their exact due date. Most are born within a week or two of that date. Your doctor may adjust your due date based on ultrasound measurements later in pregnancy.