Dating your Pregnancy
The 40 week countdown...determine your baby's due date.
You can calculate your due date by subtracting three months from the first day of your last menstrual period (LMP) and then add a week.
A pregnancy is based on being 280 days long, which is 40 weeks (more like 10 months not 9!). When we give you a due date we consider 37 to 42 weeks to be full-term, so even if your baby is born two weeks before your due date it is not considered premature.
LMP versus early ultrasound
The accuracy of using your LMP to date your pregnancy depends on having regular cycles and assumes you ovulated (released an egg) on day 14 of your cycle and conceived right away - none of which may be true. 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. If the ultrasound date is within seven days of your LMP date, we would stick with your LMP date. Ultrasounds done later in the pregnancy are less accurate for dating, so if your due date is set in the first trimester, it shouldn't be changed.
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.
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