 So establishing your due date is one of the most important things we do in a pregnancy. To figure out your due date, we usually ask about your periods, your cycle lengths, when your last period was, and we usually compare that to your first ultrasound. Based on those pieces of information, your provider should be able to give you a due date. The last menstrual period helps us figure out how far along in the pregnancy you are, and that's very important for things like genetic screening and time of delivery later. So the earlier that we can find out what your cycle was like and date your pregnancy based on that, the better. If you're unsure of the last menstrual period, or if you were taking birth control around the time you were potentially menstruating, we then frequently do a first trimester ultrasound or ultrasound early in the pregnancy to get really good dating. So those two tests done in combination give us the most accurate form of how far along a woman is.