AFP: Unusual Results and Down Syndrome

I am 33-weeks-pregnant for the eighth time. Five previous pregnancies ended in miscarriages. I am now 18-weeks along and it appears I will carry this baby boy to full term. However, our AFP results came back low indicating a 1 in 200 chance of Down Syndrome. Although no irregularities were found at the ultrasound, I am still worried.

Question:

You shouldn't be worried. You have less than a one percent chance of having a Down Syndrome baby.

These tests are not 60 percent accurate or anywhere even close to that. In fact, they are not a test for Down Syndrome at all. They are a screening test, just like using your age as a way of detecting Down's, to see who gets extra testing. They are designed to separate about five to seven percent of the pregnant women into a "high risk" group and the rest into a low risk group. 60 percent of Down's will come from the five to seven percent group while 40 percent will be scattered among the low risk group.

Overall, even in the high risk group, your chance of having a Down's baby is generally not much higher than one to two percent and often lower than that. And your chance of having a Down's baby in the low risk group is small (but not zero). Unfortunately, women belonging to the low risk group may be surprised if they do end up with a Down's baby because they assume that normal AFP results assure that this cannot happen.

With an ultrasound that reveals normal results, the risk of Down Syndrome decreases more. The numbers vary, but I use a reduction of about double -- in other words, if your risk before was 1:150, then after a detailed normal ultrasound, it would be 1:300. If the ultrasound is not performed by a perinatologist or a very experienced radiologist (most radiologists and obstetrician's will tell you that they can do a very good ultrasound; only occasionally is it true), then your risk may not fall as much. In some cases, ultrasounds reveal possible signs of Down's, and in that case, your risk may not drop much, or be higher, as well.

In your case, I doubt you should worry!

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