Overview

There are several ways for your doctor or midwife to figure out how long you have been pregnant. They help you predict when you are likely to have your baby. This is called your due date. The due date is only an estimate of when your baby will be born. Most women deliver within 14 days of their due date.^{footnote 1}

To find out how long you've been pregnant and when your baby is due, your doctor or midwife may:

• Ask you about the first day of your last period.
• Measure the size of your uterus.
• Use ultrasound testing.

Last menstrual period

The most common way to calculate your due date is to start with the first day of your last menstrual period (LMP). Add 7 days, and then count backward 3 months. For example, if your last period started on March 20, you would add 7 days to get March 27. Then subtract 3 months to get a due date of December 27.

Another way to estimate your due date is to add 40 weeks to the first day of your last period.

Size of uterus

Another way to find out your due date is by the size of your uterus. When you are about 12 weeks pregnant, your doctor or midwife can feel the top of the uterus (fundus) above your pelvis. After about 18 weeks, the distance between the pubic bone and the fundus (in centimetres) is likely to be about the same as the number of weeks since your last period. At 20 weeks, the fundus will be about as high as your belly button.

The size of the uterus is sometimes used to get a rough idea of how far along a pregnancy is. But it's not an accurate way to predict the gestational age of the growing baby (fetus). That's because there are many things that can make the fundus seem higher or lower than it really is. For example, the fetus may be in an odd position. Or you may have a uterine fibroid.

Ultrasound testing

If the first two methods can't predict your due date, you may get an ultrasound. Ultrasound tests work well to find out how long you have been pregnant, especially if they are done before 20 weeks of pregnancy.^{footnote 2} Some doctors always do an ultrasound in early pregnancy.

During an ultrasound test, a small device called a transducer is moved back and forth over your belly. The transducer sends out sound waves that bounce off the fetus. The sound waves are converted by a computer into a picture of the fetus. The picture is displayed on a TV screen.