Gestational age is the age of a pregnancy measured in weeks and days from the first day of the mother's last menstrual period (LMP). It is the standard way healthcare providers track fetal development and determine milestones like trimester boundaries and due dates.
Gestational age is most commonly measured from the first day of the last menstrual period, which is typically two weeks before conception. Early ultrasound measurements of the embryo or fetus can also be used to estimate gestational age with high accuracy, particularly in the first trimester.
The first trimester spans weeks 1 through 13, the second trimester covers weeks 14 through 27, and the third trimester runs from week 28 until birth. Full-term pregnancy is considered to be between 39 and 40 weeks of gestational age.
Gestational age is counted from the last menstrual period, while fetal age (also called embryonic age or conception age) is counted from the actual date of conception, which is typically about two weeks later. This means gestational age is always approximately two weeks more than fetal age.
A full-term pregnancy is defined as a gestational age of 39 to 40 weeks. Babies born between 37 and 38 weeks are considered early term, while those born at 41 weeks or later are late term. Preterm birth is defined as delivery before 37 completed weeks of gestation.