Gestational age (or
menstrual age) is a measure of the age of a
pregnancy where the
origin is the woman's last normal
menstrual period (LMP), or the corresponding age as estimated by other methods. Such methods include adding 14 days to a known duration since
fertilization (as is possible in
in vitro fertilization), or by
obstetric ultrasonography. The popularity of using such a definition of gestational age is that menstrual periods are essentially always noticed, while there is usually a lack of a convenient way to discern when
fertilization occurred.