Down Syndrome, Prenatal Screening, and Amniocentesis

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.

6 Responses

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  2. A very interesting free tool that computes prenatal screening for Down’s, Edwards and Patau’s syndromes is:
    http://www.sbpsoftware.com/trisomy-risk-calculator.html
    The tool use nuchal translucency for first trimester tests, and can be used to contrast with risk estimates currently provided by specialized centers.

  3. Hi,

    That was an interesting post indeed… do you know of anyone (or heard of) cases in which a woman was given a false positive for Down’s Syndrome through the amniocentesis and it was discovered? With a 1% false positive rate, there HAVE to be cases, but I am wondering if they are very hard to find at all due to the high rate of termination once women are given the positive results…

    • No, I’ve not heard of any personally, but did have some bookmarks on the subject.

      First, Examination of fetuses after induced abortion for fetal abnormality – a follow-up study: “We examined 343 mid-trimester fetuses over the last 5 years: 215 following an abnormal scan and 128 abnormal amniotic fluid or villus findings. When necessary, investigations were performed. A post-mortem examination was always required. As a result of fetal investigation, the scan diagnosis was modified or refined in 91 cases (42·3 per cent). In three of these cases, no fetal abnormality was found. For the fetuses diagnosed as abnormal by amniocentesis or chorionic villus biopsy, in one (0·8 per cent) the pre-termination diagnosis was not confirmed.”

      Second, from the BMJ (you’ll have to log in to read it, but it’s free to subscribe), “For Debate: Continuing with pregnancy after a diagnosis of lethal abnormality: experience of five couples and recommendations for management.” The parents in case 5 whose child was accurately diagnosed with a lethal abnormality said, “”We saw him. If I had had a termination we would have had nothing to remember. And I would always have wondered if the scans were wrong. I would have had that termination on my conscience for the rest of my life” (case 4). “I’d 99.9% accepted that the baby wasn’t going to live. I was always holding out a little hope. But if the pregnancy had been terminated I would have had guilt. Now I feel no guilt or remorse.”

      Third, In perfect health, the baby doctors said would be born deaf and blind …and live only a few hours — this was a case of an inaccurate ultrasound, though, not amniocentesis, which has a better than 99% accuracy.

      However, you are right, these “wrong diagnosis” would be extremely hard to find, because something like 95% of most babies diagnosed prenatally with genetic disorders or physical anomalies like Down Syndrome, anencephaly, etc., are aborted, and I would imagine that most parents would not have a post-mortem done to verify the accuracy of the test.

      So, let’s say that 4,000 babies are diagnosed by amniocentesis as having Down Syndrome every year, and 95% are aborted, leaving only 200 to be born. If the accuracy of amnio is 99.5%, then one child might be actually normal, and have been misdiagnosed. One child out of 4 million. But perhaps 19 normal babies would have been aborted. Only, most people would never know it.

      • This is a fascinating topic when you look at the figures, but a very real and personal issue for so many people.

        I have a baby girl who was diagnosed with Turner Syndrome prenatally, using amniocentesis. It was my decision not to terminate the pregnancy, however if it had been Down syndrome I most likely would have made a different decision.

        Of course I was very anxious throughout pregnancy about how severe her symptoms were going to be. When she was born, she was perfectly healthy and looked normal. The paediatrician would not have suspected there was anything wrong with her and no testing would have been done.

        That led me to start looking for examples of false-positive amnio results. You are right, very difficult to find anything. I did find one article that, although isn’t about Down Syndrome, is certainly an interesting example.

        http://www.ncbi.nlm.nih.gov/pubmed/8844007

  4. The flaw in your math is that mothers who are most at risk of having a Down syndrome child (older ones) ALSO have a much higher risk of having a miscarriage. I doubt there are statistics on amnio induced miscarriage by mother’s age, but it stands to reason that if the standard risk of miscarriage doubles from 25 to 45, the amnio rate probably does as well, so the rate for most mothers getting tested for DS is probably far higher than 1%. I have a lovely son who has Down Syndrome (we had twins and had a relatively low non-invasive risk asessment), and from my college biology classes and had no idea what a capable, delightful person he could be. He is 4, knows his numbers past 10, knows the alphabet, understands English and German and 200 signs in ASL. He is the sweetest kid you will ever meet. DSRTF is funding very promising research on drugs that promise to normalize neurotransmitter levels in DS kids and produce significant cognitive gains in our kids’ lifetimes. At a gathering of 10 DS parents recently, we had first hand knowledge of both an amnio false positive and false negative. The thought that people are willing to abort multiple healthy kids to avoid one like my son is sad given what I know now. There are tough cases with multiple severe health issues or DS and autism, but most DS kids are loving and loved members of their families who surprise those around them. Perhaps doctors spend a lot more time with the DS cases that involve severe medical issues, which leads many of them to have a negative bias about DS.

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