Gestational age is a fundamental concept and metric in obstetrics, pediatrics, and developmental biology, defining the duration of pregnancy from the first day of the mother's last menstrual period or estimated by prenatal ultrasound measurements. This concept is critical for assessing fetal growth and maturity, predicting expected delivery dates, classifying newborns (e.g., preterm, term, post-term), and evaluating perinatal health risks and outcomes in clinical practice and epidemiological research.