However, if you did not get a first trimester ultrasound for dating and an ultrasound done to look at the baby's anatomy (usually at 18-20 weeks of pregnancy) differs from your LMP date by 10 to 14 days, we would change your date to the ultrasound estimate.
Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (assuming a 28 day cycle).
There's no way to know for sure exactly when you ovulated and conceived.
A normal pregnancy can last anywhere between 38 and 42 weeks.
Note that your menstrual period and ovulation are counted as the first two weeks of pregnancy.
If you deliver on your due date, your baby is actually only 38 weeks old, not 40.
Pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period.
To find your due date, use the drop down menus below to enter the date of the first day of your last period, and click 'calculate date' - the calculator will do the rest.