The duration of a pregnancy is measured by gestational age.
Gestational age equals the amount of time elapsed since the first day of the last menstrual period.
A normal gestation lasts 40 weeks or 279 days. If delivery occurs before 37 weeks gestation, the baby is considered prematurely born. Irregular menstrual periods or first trimester vaginal bleeding can confuse gestational age estimates. To improve the accuracy of gestational age estimates, some doctors use an ultrasound examination before about 20 weeks gestation. Ultrasound findings help your doctor to confirm or correct the gestational age estimate.