<Li> Directly calculating the days since the beginning of the last menstrual period </Li> <Li> Early obstetric ultrasound, comparing the size of an embryo or fetus to that of a reference group of pregnancies of known gestational age (such as calculated from last menstrual periods), and using the mean gestational age of other embryos or fetuses of the same size . If the gestational age as calculated from an early ultrasound is contradictory to the one calculated directly from the last menstrual period, it is still the one from the early ultrasound that is used for the rest of the pregnancy . </Li> <Li> In case of in vitro fertilization, calculating days since oocyte retrieval or co-incubation and adding 14 days . </Li> <P> Childbirth on average occurs at a gestational age of 280 days (40 weeks), which is therefore often used as a standard estimation for individual pregnancies . However, alternative durations as well as more individualized methods have also been suggested . There is in any case considerable variation among individual pregnancies . </P>

Formula to calculate the expected date of delivery