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First Trimester Pregnancy – Due Date

Catherine Hansen, MD Catherine Hansen, MD February 17, 2021


"One of the most important things about pregnancy is knowing your due date. And that's important to you of course, because you can determine a lot around when you know, your baby is coming into the world, but for your healthcare provider and your healthcare team, it's also really important to know your due date, because that will help determine so many things that could be happening or should be happening incrementally throughout the pregnancy. For example, 18 to 20 weeks is when you usually have a scheduled anatomical ultrasound, and there's usually a stage around the mid 20 weeks where you would have a gestational diabetes test done and your healthcare provider can determine all kinds of things that are to happen in your pregnancy based on your due date, which ultimately implies your gestational age today. So how do you make sure that you help your healthcare provider figure out an accurate due date?

Well, you need to know the first day of your last menstrual period before you got pregnant. So hopefully you're tracking that if you're trying to get pregnant and if not, try and look on your calendar or figure out when was the first day of your last menstrual period before you got pregnant. And that will help determine your due date. There's a couple other pieces your healthcare provider will probably add, including what's called a dating ultrasound. Now that's not absolutely necessary, but the dating ultrasound, if it's done early enough to measure the baby against general tables, we can tell by the size of the baby on the ultrasound, usually only about this big, and comparing that to your menstrual period, which is still very important, and putting those pieces together to determine your due date. Once we firm up on a due date, based on your menstrual period and or your dating ultrasound, we don't change your due date. Now it is possible to deliver two weeks before to two weeks after your due date, that is entirely normal, but the due date itself stays written in ink on your prenatal record so that we can calculate based on that due date, your gestational age, and know what would be normal for that age and stage of your growing baby."

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