If the crown rump length falls outside of these five days we give you a new estimated due date based on the size of the embryo. Measuring the embryo on ultrasound is accurate to within plus or minus five days of the due date. On an early pregnancy ultrasound the embryo’s crown rump length is used to predict your estimated due date ONLY if the measurement falls outside of what your last menstrual period predicts. The accuracy of ultrasound measurements used to confirm the calculated due date will be much less accurate further into the pregnancy. Sometimes the dates can be more than a week off and sometimes even as much as 4 weeks.Īn early obstetric ultrasound performed at approximately 8 weeks where a crown rump length (the CRL) is measured precisely predicts an estimated due date accurately. It is very common when scanning an early pregnancy to find that the due date does not match the menstrual history. But we still use your LMP as a guideline because the beginning of a period is an identifiable event where ovulation and fertilisation is not. Regularity is seldom that common and there is a fair chance the calculation will be off. Normal cycles can be between 21 to 35 days with some women outside of this guideline. With so much being based on your expected due date it’s really important for it to be as accurate as possible.Ī gentle warning is that menstrual cycles are seldom that regular. It will allow you to plan ahead for your baby’s arrival and it also helps your doctor know when certain prenatal tests should be performed to make sure your baby is developing normally. Conception generally occurs shortly after ovulation (about 12 hours afterwards). If you know yours, add 266 days to your ovulation date to find your estimated due date. Or you can apply the rule in the opposite direction. Your due date is important for many reasons. LMP First day of the Last Menstrual Period EDD Estimated Due Date. The accuracy of the EDD derived by this method depends on accurate recall by the mother, assumes regular 28 day cycles, and that conception occurs on day 14 of the cycle. This is the method used by 'pregnancy wheels'. You have just attended our centre for an early pregnancy scan and your due date doesn’t match your last menstrual period (the LMP) and you are very certain of your dates. The due date may be estimated by adding 280 days ( 9 months and 7 days) to the first day of the last menstrual period (LMP).
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