Is Down syndrome a disappearing birth defect?

Fig. 1 from Collins et al. (2008)

Fig. 1 from Collins et al. (2008)

Based on their study of Down syndrome births and terminations in Victoria, Australia over a 19 year period, the study authors’ asked whether Down syndrome is disappearing. The answer is “no … not yet.”

Veronica Collins was the lead author for the study. It looked at the number of pregnancies diagnosed with Down syndrome, the number of miscarriages and stillbirths, and the number of live births with Down syndrome from 1986 to 2004. Here’s what Collins and her colleagues found.

More Down Syndrome Pregnancies Than Ever

The graph from the study shown above really tells the story.

From 1986 to 2004 the total number of Down syndrome pregnancies grew by almost 50%: going from just under 120 to over 180. The main reason cited by the authors is that in that same time the average age of pregnant women steadily increased. With Down syndrome correlating to the age of the mother, the more moms having children later in life, the more likely those pregnancies will be positive for Down syndrome.

In 1986, women over the age of 35 only accounted for 8% of all pregnancies in Victoria. By 2004, they accounted for 23%–almost a quarter of all pregnancies. As a result, the incidence of Down syndrome similarly rose, with Victoria experiencing 150% more pregnancies in 2004 carrying a child with Down syndrome than when the study period started in 1986.

More Down Syndrome Abortions Than Ever

Look at the chart above again and you’ll see the other increase. Just as pregnancies with Down syndrome increased, so, too, did the number of selective abortions. There is a near flipping in the percentage of pregnancies that were carried to full term versus those terminated.

In 1986, about 25% of all pregnancies carrying a child with Down syndrome were terminated. By 2004, with the advances in prenatal screening, just over 20% of all pregnancies were not terminated, i.e. almost 80% of all pregnancies carrying a child with Down syndrome were terminated.

The authors explain this is due to a rise in two key decisions.

In the first three years of the study, only 3% of women under the age of 35 had a prenatal diagnosis. By the end of the study, between 2002-2004, that percentage had risen to 60% of women under 35 having a prenatal diagnosis. Similarly, for women over the age of 37, the percentage increase went from 50% having a prenatal diagnosis in the late ’80′s to 80% having a prenatal diagnosis in the early 2000′s.

The authors, however, erroneously state in the study’s very first sentence:

The major factors expected to influence the epidemiology of Down syndrome (DS) are changes in the maternal age distribution in the population and the availability and use of prenatal testing.

Not quite.

While certainly the increase in the average age for mothers has increased the number of Down syndrome pregnancies, the incidence of Down syndrome births is not necessarily dependent on the availability and use of prenatal testing. The more critical decision is what expectant mothers choose to do following a prenatal diagnosis. And, the decision being most chosen, to an increasing extent, is why there are less than 50% of the children born with Down syndrome in 2004 than in 1986 despite there being 150% more pregnancies with a child with Down syndrome.

The second decision affecting the incidence of Down syndrome births is, of course, abortion:

Most cases diagnosed prenatally have resulted in termination of pregnancy. Over the entire study period, 5.3% of pregnancies with a prenatal diagnosis were not terminated.

Essentially a 95% termination rate.

Down Syndrome Disappearing

Last week’s post discussed public health professionals calling on developing countries to catch up to the first world’s use of prenatal testing and selective abortion to eliminate Down syndrome. The State of Victoria in Australia could be cited by those public health professionals as an example of their recommendations being successful.

Down syndrome is disappearing in Victoria, Australia. This when just the opposite would be occurring absent decisions to terminate following a prenatal diagnosis. There should be more children than ever with Down syndrome. Instead, there are less than half being born than in 1986.

Postscript: while the study was published 6 years ago and its end year is 2004, making the data a decade old, it remains the most current data cited as recently as last year, in this lengthy and comprehensive article on Down syndrome and prenatal testing from an Australian publication.

For other posts showing this similar dynamic of Down syndrome pregnancies rising while the number of actual births is declining, see posts at this search for “disappear” on this blog. 

Comments

  1. That was fun.

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