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  Due date
I am really confused about the 40 week gestation period. Can you explain this to me?

Many people are confused because the gestation period is calculated from the first day of your last period, when you were clearly not pregnant. Thus although the gestation period is taken as 40 weeks, in general you will only really carry the baby for 38 weeks.

To estimate your due date you:

Take the first day of your last period
Subtract three months
Add seven days
This is your estimated due date.

Clearly this is most accurate if your periods are very regular and you know exactly when your period started. As your pregnancy progresses your doctor or midwife will be able to adjust this estimated due date using the information they gain from your ultrasound, as well as by your size and the size of the baby that they can feel inside you.

Remember that only about 5% of births occur on the estimated due date, and that two weeks on either side of this date is considered normal.


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