Gestational age is counted from the first day of the last menstrual period (LMP), which is set as Week 0. Although conception usually occurs about 2 weeks after LMP, medical convention uses LMP as the starting point.
The due date is a statistical average (40 weeks from LMP) and actual delivery can differ by ±2 weeks. Ultrasound measurements of fetal size provide a more accurate estimate. Birth between 38–42 weeks is considered normal.
A baby born before 37 weeks is considered preterm and may need special NICU care. A baby born at 42 weeks or later is considered post-term; doctors often consider inducing labor due to the risk of placental dysfunction.