Length of pregnancy and due date

Length of pregnancy

The length of a pregnancy is calculated from the first day of the last menstrual period because it’s virtually impossible to know the exact moment of fertilization.

A pregnancy is generally referred to in terms of weeks. When someone says you are “20 weeks pregnant,” for example, it means 20 full weeks have gone by since the first day of your last menstrual period. The reason is simple: it is more accurate to talk about weeks than calendar months.

Your baby will be considered at term as of 37 weeks and could be born anytime between 37 and 42 weeks after your last menstrual period. Most babies are born between 39 and 41 weeks.

The 42 weeks of pregnancy (maximum length) are further divided into three trimesters of 14 weeks each. Each trimester corresponds to specific stages in the development of the fetus (see Development of the fetus).

Due date

One of the first things you’ll want to know once your pregnancy is confirmed is when the baby is due. When will the big day be?

To estimate your due date, count 280 days, or 40 complete weeks, from the first day of your last period. The expected due date is therefore only an approximate date.

An ultrasound (see Ultrasound) performed before 20 weeks of pregnancy will give you a good idea of the due date, plus or minus 7 to 10 days.