Length of pregnancy

One of the first things you’ll want to know on learning you are pregnant is when the baby is due. When will the big day be? Your baby will likely be born anywhere between 38 and 42 weeks after your last menstrual period.

To estimate your due date, count 40 complete weeks from the first day of your last period, assuming that you have a regular 28 day menstrual cycle. The expected due date is therefore only an approximate date.

If you have an irregular cycle or don’t know the date of your last period, an ultrasound performed before your 20th week of pregnancy will give you a good idea of the due date, plus or minus 7 to 10 days.

There’s a practical reason for calculating the length of your pregnancy from the first day of the last menstrual period: that’s because it’s virtually impossible to know when the exact moment of conception occurs.

The number of weeks of pregnancy therefore includes the first two weeks following your last period, even if you weren’t yet pregnant at that point. So if you are “20 weeks pregnant,” for example, it means 20 full weeks have gone by since the first day of your last menstrual period.

Your health professional will most likely refer to your pregnancy in terms of weeks. The reason is simple: it is more accurate to talk about weeks than calendar months. The 42 weeks of pregnancy (maximum length) are further divided into three trimesters of 14 weeks each.


Ultrasound: An examination using an ultrasound device that can see the embryo or fetus in the mother’s womb.