Determining Gestational Age
The duration of a pregnancy is measured by gestational age (or 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.
Some elements that can make it difficult to calculate your child's gestational age are irregular menstrual periods or first trimester vaginal bleeding. To improve the accuracy of gestational age estimates, some doctors use an ultrasound examination before 20 weeks gestation to confirm or correct the gestational age estimate.