Estimating Gestational Age

What is gestational age?

Gestational age is the age of an unborn baby, or fetus. It is measured in weeks and days and is most often 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?

There are many reasons why it is important to know a due date and how far along a pregnancy has progressed. 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. Monitoring the growth of your uterus (womb) and fetus can only be done if the age of the pregnancy is correctly known. Certain tests are done only at very specific times in the pregnancy. It is very important to know your baby's age if pregnancy complications occur and the baby needs to be delivered early. If your baby is larger or smaller than expected for its age, then your healthcare provider may recommend additional testing. 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, your cycles are around every 28 days, 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. Sometimes more than one ultrasound examination may be needed if your baby is growing faster than expected.

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 are 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.

Developed by Phyllis G. Cooper, RN, MN, and RelayHealth.
Published by RelayHealth.
© 2009 RelayHealth and/or its affiliates. All Rights Reserved.