First trimester dating ultrasound

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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.

Standard Ultrasound – Traditional ultrasound exam which uses a transducer over the abdomen to generate 2-D images of the developing fetus.

Advanced Ultrasound – This exam is similar to the standard ultrasound, but the exam targets a suspected problem and uses more sophisticated equipment.

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.

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.

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Transvaginal scans may be used early in pregnancy to diagnose potential ectopic or molar pregnancies.

This is used to help assess suspected congenital heart defects.

The traditional ultrasound procedure involves placing gel on your abdomen to work as a conductor for the sound waves.

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.

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