I don't know what places the author's referring to, but according to one study, most fetuses diagnosed with Down Syndrome in Europe [0] and the United States [1] are aborted. The rate of live births has remained steady, but apparently it would be significantly higher if not for prenatal screening [2]. This discrepancy isn't very mysterious, nor is it really an absence, so maybe the GP has something else in mind?

[0]: https://www.ncbi.nlm.nih.gov/pubmed/10521836

[1]: https://www.ncbi.nlm.nih.gov/pubmed/22418958

[2]: http://onlinelibrary.wiley.com/doi/10.1002/pd.2702/full