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Ultrasound accuracy in dating pregnancy

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For many women, especially after 8 weeks gestation, sufficient information about the baby may be obtained with transabdominal ultrasound only.However, in the early pregnancy, the developing embryo is very small (at 6 weeks gestation, the baby is only 5-9mm long) and a transvaginal ultrasound may be required to get a better image of the baby.

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Before twelve weeks, it’s difficult to measure much more. In the second trimester, (past the first 12 weeks), the other measurements can be added.However, if you have a larger than average baby, the ultrasound will apply the husky figures to the “normal” measurements.This creates error in that the baby will compute out to be further along than he or she really is.Therefore, measurements taken later on, when babies begin to grow at different rates among pregnancies, yield increasingly inaccurate dating of pregnancy.Although the dating of pregnancy accuracy takes a dive, still ultrasound is indispensable for the other uses.When an ultrasound is performed, measurements of the head, abdomen, thigh, and amount of amniotic fluid are done.

These measurements are computed automatically in the ultrasound machine’s software.

For instance, say you normally have eight and nine pound babies.

Then your baby at 37 weeks will have bigger measurements than babies destined to weigh seven or eight pounds at birth.

The simple rule in ultrasound is that when the due date based on ultrasound doesn’t vary from the mother’s dates by more than a week, stick with the mother’s dates; if the ultrasound disagrees by more than a week to ten days, it becomes wiser to rely on the ultrasound.

This doesn’t apply later on in pregnancy for the reasons above. If two ultrasounds one month apart determine coinciding due dates, especially if they agree with the mother’s date based on a last period, the accuracy can be within a couple of days. Babies have their own clock and can come anywhere from three weeks before this exquisitely determined due date till two weeks after.

So in this arithmetic test, the baby sets the curve.