Pregnancy Due Date Calculator
Use this calculator to work out when your baby is due. You will need to give us the date of the first day of your last period. Because pregnancy normally lasts from 35 to 40 weeks from the time of conception we can use this date to work out when you can expect your baby to be born.
Choose the date of the first day of your last period:
Most women's menstrual cycle is not exactly 28 days (that's just an average), so
please adjust the number of days cycle length if necessary:
How is the due date calculated?
A pregnancy that goes to full term normally lasts from between 38 and 42 weeks. The formula used to calculate the due date is based on original observations by Franz Naegele, a German professor and obstetrician who ran a hospital in Dusseldorf in the 19th century. His rule was to work back three months from the day of a woman's last period and add a year (effectively adding nine months). The assumption is that ovulation and fertilization occur on day 14 of a 28 day menstrual cycle. This is equivalent to 280 days since the start of the last period, before allowing for cycle length adjustments.
In 1990, new research suggested that the average pregnancy lasts 288 days from the last period for Caucasian women. It also indicated that other factors affect the gestation period including the mother's weight, age, lifestyle, number of previous pregnancies, and ethnicity. With this formula, about 65% of babies are born within ten days of the predicted date, with 80% being within the 38-42 week band. This still leaves considerable uncertainty about when the baby will be born, so the predicted due date should really be viewed as only a guide.
Nowadays a more accurate due date can be obtained by means of ultrasound scans. This allows the radiographer to measure the size of the developing embryo in order to predict the likely birth date. Surprisingly, the accuracy of the technique (to within about 6 days) is greatest during the first trimester (12 weeks) of pregnancy, and actually becomes less accurate as the pregnancy continues.