<P> Due date estimation basically follows two steps: </P> <Ul> <Li> Determination of which time point is to be used as the origin for gestational age . This starting point is the woman's last normal menstrual period (LMP) or the corresponding time as estimated by a more accurate method if available . Such methods include adding 14 days to a known duration since fertilization (as is possible in in vitro fertilization), or by obstetric ultrasonography . </Li> <Li> Adding the estimated gestational age at childbirth to the above time point . 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 . </Li> </Ul> <Li> Determination of which time point is to be used as the origin for gestational age . This starting point is the woman's last normal menstrual period (LMP) or the corresponding time as estimated by a more accurate method if available . Such methods include adding 14 days to a known duration since fertilization (as is possible in in vitro fertilization), or by obstetric ultrasonography . </Li> <Li> Adding the estimated gestational age at childbirth to the above time point . 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 . </Li>

When do many obstetricians date the onset of pregnancy from