Dating the pregnancy

Setting a reliable EDD is one of the important functions in AN care.
Precise dating is important in preterm gestations, prolonged pregnancies and a number of different screening tests.
The EDD can be calculated from the first day of LMP by adding 7 days and 9 months. However, this method assumes menstrual cycle to be 28 days cycle with ovulation on day 14 and the woman must remember her LMP accurately. In reality, most women have not 28 days cycles and the timing of ovulation is variable within a cycle. Menstrual dates are not accurate in 25-40% of women.
Dating by an ultrasound scan in the first trimester must be done ideally between 10 and 14 weeks, and EDD set by this scan should be used in preference to menstrual EDD.
CRL crown rump length is used up until 13weeks+6days with accuracy ±4days.