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 got 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 last started your period. An early exam of the uterus and an early positive pregnancy test also can 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 the 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 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 pregnancy.