How is gestational age defined?

Gestational age is defined based on the first day of the last menstrual period (LMP) because this date is a reliable and standardized reference point for estimating the duration of a pregnancy. Medical professionals typically calculate gestational age from this date since most women can accurately recall when their last period began, and it provides a consistent method regardless of when conception actually occurred.

Estimating gestational age starting from the LMP allows for a more standardized approach, as fertilization often occurs approximately two weeks after the onset of the menstrual cycle. This method provides a framework that aligns with the 40-week duration often used to denote a full-term pregnancy.

Other options, such as the date of fertilization, the date of diagnosis, or the date of implantation, can be less reliable or not commonly used in clinical practice because they can vary significantly among individuals and do not offer the same level of consistency for tracking pregnancy progression.