Baby’s age in weeks from the first day of the mother’s last menstrual period before delivery. The duration of a pregnancy is measured by gestational age. Gestational age is basically a measure of the length of time that a baby spends in the womb. It is generally calculated as the time from the first day of the last menstrual period to the day of birth. Conception occurs on about day 14 after the first day of the last menstrual period. However, this date is not usually known. The first day of the last menstrual period usually is known. Therefore the beginning date for the pregnancy is the first day of the last menstrual period.
A normal gestation lasts 40 weeks or 280 days. If delivery occurs before 37 weeks gestation, the baby is considered prematurely born. By this definition, about 11% of all babies are born prematurely. Irregular menstrual periods or first trimester vaginal bleeding can confuse gestational age estimates. To improve the accuracy of gestational age estimates, many doctors use an ultrasound examination before about 20 weeks gestation. Ultrasound findings help your doctor confirm or correct the gestational age estimate.