Gestational Age
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 the age of the baby.
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 first day of the mother’s last menstrual period. Using ultrasound to measure the baby and determine gestational age is most accurate between the 8th and 18th weeks of pregnancy. It becomes less accurate later in pregnancy. The most accurate way to determine gestational age is using the first day of the woman’s last menstrual period and confirming the 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 2 weeks 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 last period. This is just an estimate since only about 5% of babies are born on their estimated due date.
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