How is gestation typically measured?

Gestation is typically measured from the first day of the mother's last menstrual period (LMP) because this provides a consistent reference point for estimating the length of pregnancy. This method is widely used in obstetrics because the exact date of fertilization is often difficult to determine, while the start of the last menstrual period is usually well-documented and can be accurately identified by the mother.

By using the first day of the LMP, healthcare providers can estimate the due date and track the progression of the pregnancy using standard gestational age milestones. This approach accounts for the average 28-day menstrual cycle and is essential for prenatal care and monitoring the health of both the mother and the fetus.

Other methods of measuring gestation, such as calculating from the baby's birth date or the start of the third trimester, do not provide accurate estimates for determining the gestational age. While the day of fertilization might seem like a logical starting point, it is often not known precisely even in clinical settings. Therefore, the first day of the last menstrual period remains the most practical and widely accepted method in medical practice.