gestationalage

Gestational age is a measure of how far along a pregnancy is. It is typically calculated from the first day of the last menstrual period (LMP) and is expressed in completed weeks and days. Because conception usually occurs about two weeks after the LMP in a standard 28-day cycle, gestational age can be viewed as roughly two weeks greater than the age of the conceptus or embryo.

Most pregnancies are dated using the LMP, with due date estimated by Naegele's rule: due date = LMP plus 280 days. However, this method assumes accurate recall and regular menstrual cycles. When those conditions are uncertain, obstetric ultrasound dating provides a more precise estimate, especially in the first trimester, by measuring the crown-rump length. Later in pregnancy, fetal biometrics (such as head circumference, abdominal circumference, and femur length) refine gestational age but are inherently less precise.

Gestational age is used to classify pregnancy stages and to time interventions and screenings. Common categories are preterm (<37 weeks), term (37 to 42 weeks, with early term usually 37 to 38 weeks 6 days, full term 39 to 40 weeks 6 days, and late term 41 to 41 weeks 6 days), and post-term (42 weeks and beyond).

Clinically, gestational age guides decisions about prenatal testing, fetal growth assessment, and monitoring for complications such as preterm labor or fetal growth restriction. It is distinct from fetal or embryonic age, the latter referring more directly to the time since conception; gestational age remains the standard term in obstetric care.