I recently came across this article which said that there are “160 healthy babies lost for every 50 Down’s cases detected with amniocentesis.” That’s an exaggeration which reminds me of the recent post I wrote about Skewing the Statistics. Here are the facts: amniocentesis is recognized as having a 1 in 200-400 risk of miscarriage; and the risk of having a baby with a genetic condition such as Down Syndrome is about 1 in 1000. So, if every pregnant woman were given an amniocentesis to accurately assess whether the baby had Down’s, then (estimating 4 million births per year) about 10,000-20,000 babies would be miscarried as a result (most of them healthy), while only about 4,000 babies would actually have Down Syndrome or other similar genetic condition. But not every pregnant woman is given an amnio. Typically, women are offered an amnio if they are at risk of having a baby with a condition that an amnio can predict — for instance, if they are over a certain age, or if a screen such as the nuchal translucency test indicates that they are at high risk for having a baby with Down Syndrome or some other condition. However, if doctors are being encouraged to offer women amniocentesis as a matter of course, then, yes, that article would be correct, and an extraordinarily high number of babies will be miscarried that otherwise would be carried to term.
There are various screening tests that can be performed prenatally, but their accuracy varies, depending on the type of the screen and what is being screened for. In looking at screens such as the nuchal translucency test, they typically miss about 15-25% of babies who have Down Syndrome, and inaccurately flag about 5% of babies as being “high risk” for Downs when they’re not. So, if all (4 million) pregnant women got the NT test, and 85% of DS cases were accurately identified (3400), and 5% of women are inaccurately identified as high-risk (200,000), and all “positive” results were further tested by amniocentesis with a 1:300 risk of miscarriage, then 11 babies with Down Syndrome will be miscarried, and about 667 babies without Down Syndrome will be miscarried. So 600 babies with Down Syndrome would be missed, 3400 would be discovered, and 667 normal babies would die by miscarriage. However, these results will be different, depending on the real-life risk of miscarriage from amniocentesis (it varies, based on the provider’s skill and how many he’s done, as well as when in pregnancy it’s done), as well as on the accuracy of the NT test. If, as the author of the above-mentioned article says, the NT screening is not as accurate in the real world as it seemed to be in the research environment, and there is an even higher false-positive rate, then more babies will be miscarried due to invasive tests such as amniocentesis which would not have been done had their mothers received an accurate negative result.
This study was interesting, in that it talked about tests other than nuchal translucency, to determine women at higher risk for having a baby with Down Syndrome (or other condition). (Here is the table provided in the study which shows 7 different ultrasound markers used to identify babies at higher risk of having Down Syndrome.) If all babies found to have “choroid plexus cyst” were subsequently given an amniocentesis to test for Down Syndrome, then there would be so many miscarriages due to amniocentesis, that for every ten babies positively identified to have Down Syndrome, another forty-three normal babies would die. As maternal age increases, so does the likelihood of having a baby with Down Syndrome; if only women at high risk are screened, then the rate of fetal losses drops to just under two dead normal babies for every baby found to have Downs. (Let me say that this study does not look at nuchal translucency, which has recently been recommended to be offered to every woman, although not every doctor has the expertise to perform it. This study is for second-trimester ultrasound screening, and the nuchal translucency is a first-trimester test.)
Having a “false positive” rate of 5% sounds pretty good, until it appears that if 5% of the population is wrongly identified as being at high risk for having a certain condition, then that equals 200,000 women and babies. That’s a lot!
I encourage you to read the study I linked to, especially the comment section, because it has a lot more information, but here is one quote:
The use of the ultrasonographic markers as an indicator for invasive testing with amniocentesis will lead to an increase in the number of unaffected fetuses lost as a complication of the procedure…. this highlights an important potential harm associated with a test often considered risk-free. If, for example, identification of an echogenic intracardiac focus is used as a basis for offering amniocentesis to pregnant women at low risk of carrying an affected fetus, 2 unaffected fetuses will be lost as a complication of amniocentesis for each correctly identified Down syndrome case. Additionally, because the false-positive rate is 1% or greater for most of the markers, when all of these markers are used in aggregate, the false-positive rate may approach 10% or more, leading to much needless anxiety throughout pregnancy and beyond.
In a previous poll, I asked how many babies with Down Syndrome were aborted, when their condition was discovered in utero. The answer is 85% or more. Most studies show at least 90-95% termination rates, but I won’t link to all of them.
Filed under: abortion, miscarriage, pregnancy, studies & stuff, ultrasound Tagged: | abortion, choroid plexus cyst, down syndrome, echogenic bowel, echogenic cardiac focus, femur length, genetic defect, genetic disorder, humerus length, NT test, nuchal fold, nuchal translucency, second-trimester screening, thickened nuchal fold, trisomy 13, trisomy 18, trisomy 21, ultrasound