In cases where the date of conception is known precisely, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception. Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the the first day of the last menstrual period).
Does ultrasound show due date?
Due Date Accuracy Evidence suggests that, in the first 20 weeks of pregnancy, the first ultrasound may be the most accurate tool for calculating a fetuss gestational ages. If you cant remember the date of your last menstrual period date, an early ultrasound can give you a fairly reliable due date.
Can an ultrasound tell you how far along you are?
One of the biggest benefits of an ultrasound is that it can estimate how far along you are in your pregnancy (gestational age) down to just a few days. Ultrasounds estimate how far along you are in your pregnancy by comparing the growth of the fetus to established growth rates for babies around the world.