How the due date is estimated
Doctors usually count pregnancy as 40 weeks (280 days) from the first day of your last menstrual period (Naegele's rule), assuming a 28-day cycle. If your cycle is longer or shorter, the date is adjusted by the difference. From a known conception date, the due date is 266 days later.
Only an estimate
Only a small share of babies arrive exactly on their due date; most are born within about two weeks either side. An early ultrasound scan usually gives the most accurate dating, and your doctor may adjust your due date based on it.
This calculator is for general information only and is not medical advice. Please follow the guidance of your doctor or midwife and attend all antenatal check-ups.