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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.
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. 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! 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.
Ultrasounds performed during the first 12 weeks of pregnancy are generally within 3 - 5 days of accuracy.Usually the expected date of delivery (EDD or EDC) is calculated from your last menstrual period - if the early dating scan calculates the EDD to be within 5 days of the EDD from your last menstrual period.The EDD from the early dating scan is used - if the last menstrual period is not known or is unreliable, or the dating scan differs from the last menstrual period dating by more than 5 days.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.Your access to the NCBI website at gov has been temporarily blocked due to a possible misuse/abuse situation involving your site.