How is the due date calculated?
It uses Naegele's rule: the first day of your last period + 280 days (40 weeks). If your cycle is longer or shorter than 28 days, the date is shifted by the difference. If you know the conception date, the due date = conception + 266 days.
Pregnancy weeks are counted from the first day of the last period, not from conception, so they are about two weeks more than the baby's actual age.
Only about 4% of babies arrive exactly on the due date; most come between weeks 37 and 42.
FAQ
How accurate is the due date?
It is an estimate. Most births happen within two weeks either side, and your doctor may adjust the date after an early scan.
What are the trimesters?
The first runs to the end of week 13, the second from week 14 to 27, and the third from week 28 until birth.
What if my periods are irregular?
Dating from the last period is less accurate. A first-trimester ultrasound is the most accurate way to date a pregnancy.
This calculator is general information only and is not a substitute for medical care. See your doctor to confirm how far along you are and your due date.