Courtesy of the American Pregnancy Association, Barefoot & Pregnant
Gestational age, or the age of the baby, is calculated from the first day of the mother's last menstrual period. Since the exact date of conception is almost never known, the first day of the last menstrual period is used to measure how old the baby is.
Use our Pregnancy Calculator to calculate these dates for you.
Calculating Gestational Age:
Last Menstrual Period: If the mother has a regular period and knows the first day of her last menstrual period, gestational age can be calculated from this date. Gestational age is calculated from the first day of the mother's last menstrual period and not from the date of conception.
Ultrasound: The baby can be measured as early as 5 or 6 weeks after the mother's last menstrual period. Measuring the baby using ultrasound is most accurate in early pregnancy. It becomes less accurate later in pregnancy. The best time to estimate gestational age using ultrasound is between the 8th and 18th weeks of pregnancy. The most accurate way to determine gestational age is using the first day of the woman's last menstrual period and confirming this gestational age with the measurement from an ultrasound exam.
Calculating Conception Date:
In a Typical Pregnancy: For a woman with a regular period, conception typically occurs about 11-21 days after the first day of the last period. Most women do not know the exact date of conception, and their conception date is merely an estimate based on the first day of their last period.
Special Cases: Women who undergo special procedures such as artificial insemination or in vitro fertilization typically know the exact date of conception.
Calculating Due Date:
Estimated Due Date: Based on the last menstrual period, the estimated due date is 40 weeks from the first day of the period. This is just an estimate since only about 5% of babies are born on their estimated due date.
Difficulties in Determining Gestational Age:
Last Menstrual Period: For women who have irregular menstrual periods or women who cannot remember the first day of their last menstrual period, it can be difficult to determine gestational age using this method. In these cases, an ultrasound exam is often required to determine gestational age.
Baby's Growth: In some cases it is difficult to determine the gestational age because the baby is unusually large or small. Also, in some cases the size of the uterus in early pregnancy or the height of the uterus in later pregnancy does not match the first day of the last menstrual period. In these cases as well, it is difficult to obtain an accurate gestational age.