How it works
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception, so a full term is 40 weeks (280 days). This is Naegele's rule, the method most clinicians use for an initial estimate.
If your cycle is longer or shorter than 28 days, ovulation usually shifts by the same amount, so the due date is moved later or earlier to match. If you know the conception date, the due date is 266 days (38 weeks) after it.
An early ultrasound often refines the date, and your care provider's estimate takes priority. Results are estimates, not medical advice.
Formula
Due date = LMP + 280 days + (cycle length − 28) · or conception date + 266 days
Questions people ask
How accurate is a due date?
It is an estimate. Only about 4% of babies are born on the exact day; roughly 80–90% arrive within two weeks either side. A first-trimester ultrasound is the most accurate way to date a pregnancy.
Why is pregnancy 40 weeks if conception happens at week 2?
By convention, pregnancy is dated from the first day of the last period because that date is usually known. Conception typically happens around two weeks later, so the baby's actual age is about 38 weeks at the due date.
What if my periods are irregular?
LMP-based dating is less reliable with irregular cycles. An ultrasound measurement is usually used instead to set the due date.