Early ultrasound gestational age dating
This estimation assumes that conception occurs on day 14 of the cycle.
Also, vaginal bleeding occurs during 15-25% of first trimester pregnancies, The mean pregnancy length has been estimated to be 283.4 days of gestational age as timed from the first day of the last menstrual period and 280.6 days when retrospectively estimated by obstetric ultrasound measurement of the fetal biparietal diameter (BPD) in the second trimester.
The most common system used among healthcare professionals is Naegele's rule, which estimates the expected date of delivery (EDD) by adding a year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period (LMP) or corresponding date as estimated from other means.
Using gestational age, births can be classified into broad categories: Using the LMP (last menstrual period) method, a full-term human pregnancy is considered to be 40 weeks (280 days), though pregnancy lengths between 38 and 42 weeks are considered normal.
Furthermore, actual childbirth has only a certain probability of occurring within the limits of the estimated due date.
A study of singleton live births came to the result that childbirth has a standard deviation of 14 days when gestational age is estimated by first trimester ultrasound, and 16 days when estimated directly by last menstrual period.