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In the past, doctors used the first day of the mother's last menstrual period to calculate due dates. This was because the exact date of conception was hardly ever known for sure. These days, with pregnancy tests that can confirm pregnancy only a couple of days after conception, it might be easier to make an accurate guess. Keep in mind that few babies ever come right on schedule.
To calculate your due date from conception, use these simple rules:
This makes sense because conception occurs when women ovulate, and this happens in the middle of a monthly menstrual cycle. Subtracting two weeks if you use the date of conception from the calculation based on the first day of a period is a good rule of thumb.
For example, you might have had your last period on Valentine's Day. That means you probably conceived right around the first of March. You should be prepared to have your baby in time for Thanksgiving.
What if you do not know your conception date or the first day of your last period?
Women have been known to lose track of these things, but an ultrasound as soon as five or six weeks after conception can usually help estimate the baby's gestational age very accurately.