On Amniocentesis and Down’s Syndrome

The image and the paragraph below are taken from a pamphlet designed to inform women about prenatal tests to detect possible birth defects. Please read this carefully, noting warnings and side effects. After you read it, I will make a few comments and provide additional information.

from an official pamphlet of the American College of Obstetricians and Gyncecologist

Amniocentesis

Amniocentesis usually is done at 15–20 weeks of pregnancy. To perform the procedure, a doctor guides a thin needle through your abdomen and uterus. A small sample of amniotic fluid is withdrawn and sent to a lab.

In the lab, cells from the baby that were taken from the amniotic fluid are grown in a special culture. This usually takes about 10–12 days. Next, the chromosomes in these cells are studied under a microscope. This shows if there is an extra chromosome (as in Down syndrome) or other chromosomal defects. The alpha-fetoprotein (AFP) level in the amniotic fluid also can be tested to help determine if the fetus has a neural tube defect.

Complications from amniocentesis are uncommon. Side effects may include cramping, vaginal bleeding, infection, and leaking amniotic fluid. There is a slight chance of miscarriage (less than 1%) as a result of amniocentesis…

more via ACOG

There is no mention that amniocentesis typically only detects about 30% of the fetuses that are born with Down’s. Also, approximately 15% of the fetuses diagnosed with Down’s by this test are incorrectly identified and are actually “normal”. Another important contextual point is that out of 10,000 pregnancies there is are 35 fetuses with Down’s. To put it differently, out of every 280 pregnancies, there is one Down’s fetus.

Assuming the conservative risk of miscarriage due to amniocentesis to be .5% (1 out of every 200 pregnancies tested), then if 100,000 women receive this test there will be 500 miscarriages. Out of 100,000 tests, approximately 340 actual fetuses will be discovered, and 60 fetuses will be incorrectly detected, and 6,660 fetuses with Down’s will be declared normal.

I don’t know about you but to me the decision to utilize this test doesn’t look so clear-cut anymore.

here is the graphic on which I base my information. The source is at Down’s Syndrome Online

Down syndrome screening process - possible outcomes and measures of accuracy

 

My main point is that the ACOG pamphlet seems designed more to sell services that to inform.

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