The accurate determination of a patient's "due" date, referred to by doctors and midwives as the EDC (Estimated Date of Confinement) or EDD (Estimated Date of Delivery), is very important for a variety of reasons. The timing of certain tests, the monitoring of the baby's growth, and the correct diagnosis of premature labor, or being truly "overdue," (postdates), as well as many other situations that arise in the course of a typical pregnancy, all depend on a correct determination of the EDC for appropriate management. Show
In the past, the EDC was calculated by using Naegele's Rule, which determined the date by subtracting 3 months from the 1st day of the last period and then adding 7 days. In fact, when a woman who has very regular, "textbook" 28 day cycles presents for prenatal care, this often turns out to be the correct EDC more often than not. Other information used to calculate the gestational age, or the number of weeks and days from the first day of the last menstrual period (LMP), includes the size of the uterus on pelvic bimanual examination. However, it is extremely common to encounter patients who have irregular or infrequent menstrual cycles, or have fibroid tumors that cause their uterus to feel enlarged on pelvic exam, or who conceive shortly after a pregnancy ends (without ever actually having had a period after the last pregnancy), or who got pregnant while taking birth control pills, and these situations often render the above methods useless and misleading when trying to figure out a reliable EDC. This is where our wonderful ultrasound machines can make a crucial difference. With today's modern equipment, we can obtain very reliable images and measurements of even very early pregnancies, sometimes even seeing a heartbeat as early as 5-6 weeks! The most common misconception we encounter almost daily, has to do with how accurate ultrasounds are in fixing the EDC at different stages of pregnancy. Many patients believe that the later in pregnancy an ultrasound is performed, the more reliable the EDC calculated by the ultrasound is. Perhaps this is because patients reason that since we can see more detail on ultrasound the further along one is (where we can see noses, ears, fingers, the chambers of the heart, etc.) the numbers should be more accurate. The truth is actually the exact opposite! Just as children and adults of the same age can vary dramatically in height and weight, when a baby is past the first trimester he/she also can be larger or smaller than average, depending on genetics (how tall the parents are, how much they weighed at birth), nutrition, and coexisting medical conditions (diabetes, high blood pressure, etc.). However, in the first trimester there is very little variation in fetal size, and so it turns out that an ultrasound done between 7 and 13 weeks is the most accurate. How accurate, you say? Compare the maximum error that each ultrasound can potentially have, and you'll see what I mean:
This means that a pregnancy for which no period dates are available, and who did not get an exam or ultrasound until the third trimester, can have an EDC which could conceivably fall within a range as broad as 42 days! Talk about uncertainty. The main situation where this can cause a bit of friction between provider and patient is when the patient has uncertain dates, and/or a late ultrasound, and feels like she's close enough to full term to ask for her labor to be induced, yet she has a cervix which is unripe, or unfavorable for induction. This can sometimes lead, if one isn't careful, to unnecessary induction of labor, which can result in unnecessary cesarean section, or delivery of a premature baby, who then requires transfer to a special care nursery. Both these situations are considered largely avoidable. In our practice, the EDC will be determined at the first prenatal visit whenever possible, and subsequent ultrasounds, which often will report a different (often earlier) EDC will not override the first EDC. In fact, the only time that this doesn't apply is when the date of conception is known without doubt, as when an infertility patient conceives with timed intercourse or artificial insemination. A due date calculator tells you when to expect your baby. You can calculate your due date using the date of your last period or the date you got pregnant. Your due date helps you plan and it helps your provider give you and the fetus the best care. But it’s only an estimate. Your due date could change, or you may have your baby on another day.
OverviewWhat is a due date calculator?A due date calculator helps you estimate when your baby will be born. By calculating your due date, you’ll have an idea of your baby’s birthday. But keep in mind that your due date is only an estimate. Very few babies are actually born on the day they were due. Knowing your due date is important for several reasons. It helps you plan and prepare for your baby’s arrival. It also gives your provider essential information they need to monitor your health and the developing fetus. You can calculate your due date at home using a couple of different methods. But these methods aren’t always accurate. When you visit your provider, they may give you a different due date based on the size of the fetus on ultrasound and the date of your last period. What is a due date?It takes about 38 weeks (266 days) for a fetus to develop fully in the uterus. Your due date is the day you’re expected to give birth based on this 38-week gestation process. But keep in mind that healthcare providers consider a pregnancy to be full-term anywhere between 37 and 42 weeks. Babies can be born at any time, and many factors play a role in determining when you go into labor and when your baby will actually enter the world. Test DetailsHow do I calculate my due date?You can calculate your due date by using: The date of your last periodYou can calculate your due date if you know the first day of your last menstrual period (LMP) before you got pregnant. Your due date is 40 weeks after the day you started your last period. This method is a good way to estimate when your baby is due because many women people the day they started their period. But it might not always be accurate, especially if you have an irregular menstrual cycle or are on birth control. The date you conceived (got pregnant)If you know the exact day you conceived, you can use that information to figure out when your baby is due. Count ahead 38 weeks from the day you got pregnant. That day is your due date. This method is only accurate if you’re sure about the day you got pregnant. You may only know the specific day you got pregnant if you know when you ovulated. Ovulation happens when your ovaries (part of your reproductive system) release an egg. Usually, women ovulate about 14 days after the first day of their period. You can track your ovulation by:
But even if you know when you ovulated, it can be difficult to know exactly when pregnancy occurred. Sperm can live inside your fallopian tubes (the tubes that carry eggs to your uterus) for up to five days. You can only get pregnant about 12 to 24 hours around the time that you ovulate. So, if you ovulated a few days after you had sex, you may have become pregnant then. How do I calculate my due date if I had IVF?If you used embryo cryopreservation and in vitro fertilization (IVF) to get pregnant, you can use your IVF transfer date to calculate your due date. If your provider transferred the embryos five days after they retrieved your eggs, count ahead 261 days from the transfer date. If your provider transferred the embryos three days after egg retrieval, count ahead 263 days to determine your due date. On your transfer day, your provider will calculate your due date and share this information with you. How accurate is a baby due date calculator?A due date calculator is only an estimate. Your healthcare provider will examine you and your developing fetus to confirm your due date. To get a more accurate idea of when your baby will be born, your provider will perform an ultrasound. During an ultrasound, your provider measures the fetus. These measurements help your provider determine gestational age and confirm your due date. Why is it important to calculate my due date?It’s important to know your due date for planning purposes — knowing your expected delivery date helps you prepare to welcome your baby. You’ll have time to pack for the hospital and get the things you need for the baby, like a car seat. If you have other children, counting down the days until the new baby arrives can help siblings prepare for a new family member. Knowing when you’re due also allows you to get appropriate prenatal care. This includes taking prenatal vitamins and getting prenatal screenings at the right times during your pregnancy. Your provider will also measure the fetus's size and position throughout your pregnancy to ensure that it's growing properly. How do I calculate my due date if I don’t know the date of my last period?If you don’t know the date of your last period or when you conceived, talk to your provider. They will do an exam, which will likely include a prenatal ultrasound, to determine how far along you are in your pregnancy. What is a reverse due date calculator?A reverse due date calculator is helpful if you already know your due date and you want to find out when you got pregnant. To find out when you conceived, just subtract 266 days from your expected delivery date. You probably conceived around that day. Results and Follow-UpWhat do I need to know about using a pregnancy due date calculator?A due date calculator may not be completely accurate. This is especially true if you aren’t exactly sure when you had your last period, you have irregular periods or you don’t know when you conceived. You should see your provider for a more precise test, such as an ultrasound, to confirm your due date. Can my due date change?Due dates are only an estimate. Providers consider a pregnancy to be full term after 37 weeks. So, keep in mind that you may very well deliver your baby before or after your due date. When you see your provider for an ultrasound, they may tell you that your due date has changed. Sometimes fetal measurements show that you aren’t as far along as you thought. Or, if the fetus is bigger than expected based on your due date, your provider may adjust your due date to make it a few days earlier. This is all perfectly normal. How does carrying multiples affect my due date?Expecting twins or triplets is different than expecting one baby in many ways, including the due date. If you’re pregnant with multiples, your doctor will use your last menstrual period and an ultrasound to determine your due date. However, every pregnancy is unique and your doctor may recommend delivery prior to the estimated due date. Talk to your provider about when you can expect to deliver twins or triplets. When should I call my doctor?Call your provider if you have any questions about your due date or fetal growth. If you think your due date is incorrect, mention this to your provider. A note from Cleveland Clinic A due date calculator helps you plan for your baby’s arrival. Keep in mind that a due date calculator is only an estimate. You’ll need to see your provider to confirm the date. Your due date may change when you have a prenatal ultrasound. Your provider may even change your due date more than once during your pregnancy, but it usually only changes by a few days. Knowing when to expect your new family member allows you to prepare mentally and physically. An accurate due date also helps your provider monitor your health and the developing fetus. Share Email Print Last reviewed by a Cleveland Clinic medical professional on 10/07/2021. References
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To determine the clients edc, which day of the menstrual period will you ask?
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