How to Calculate Gestational Age

What is gestational age?
Gestational age is the age of an unborn baby, or fetus in the uterus. It is measured in weeks and days and is based on the date of your last menstrual period.

Your healthcare provider will talk about your pregnancy in terms of weeks, not months. There are 3 stages of each pregnancy, called trimesters. The first trimester is from the 1st week through the 13th week. The second trimester is from the 14th through the 27th week, and the third trimester is from the 28th week to delivery.

Why is it important to know the gestational age?
At specific stages of the pregnancy, certain things are expected. For example, first hearing the heartbeat, or feeling the baby move, is expected to happen at a certain time during the pregnancy. How the pregnancy is going can be judged as normal or abnormal only when the age of your baby is correctly known. It is very important to know your baby’s age if problems occur and the baby needs to be delivered early. It is also important to know when a baby is overdue so the health of the baby can be more carefully watched.

How is gestational age calculated?
The age of your baby and your due date may be calculated from the date of your last menstrual period. If your periods were regular before you became pregnant, and you are sure of the first day of your last period, your due date is estimated to be 40 weeks from the day you started your last period. An early exam of the uterus and an early positive pregnancy test also help determine your baby’s age.

Ultrasound can be used to confirm your baby’s age. The baby can be measured with ultrasound as early as 5 or 6 weeks after your last menstrual period. This method is most accurate in the first half of the pregnancy. The best time to date a pregnancy with ultrasound is between the 8th and 18th weeks of pregnancy.

If you have had a special procedure to become pregnant, such as artificial insemination or in vitro fertilization, you will know when your baby was conceived. In these cases there is no doubt about a baby’s age.

It can be hard to determine accurately a baby’s age and your due date if:

  • Your periods were irregular.
  • You cannot remember the date of your last period.
  • The baby is unusually large or small.
  • The due date based on the last menstrual period does not correspond with the size of the uterus early in pregnancy or with measurements of your uterus later in the pregnancy.