Dating your pregnancy
Please remember that your due date is only an estimate.Every pregnancy is unique and your baby will come when it's ready.Because few women know the exact day they ovulated or conceived, an ultrasound done in the first trimester of pregnancy has been shown to the be the most accurate way to date a pregnancy.If an ultrasound date in the first trimester differs from your LMP date by seven days or more, we would go with the ultrasound.There's no way to know for sure exactly when you ovulated and conceived.
So you will be two weeks further along in your pregnancy if it's calculated according to your LMP rather than your estimated date of conception.
Most are born within a week either side of the estimated due date.
A normal pregnancy can last anywhere between 38 and 42 weeks.
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).