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Dating a Pregnancy

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.

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:

  • 1st Trimester (7-13 weeks) : plus or minus 5 days
  • 2nd Trimester (14-28 weeks) : plus or minus 10-14 days
  • 3rd Trimester (29-40 weeks) : plus or minus 21 days!

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.








Call Cartersville Ob/GYN today (770) 386-4824

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