Given we can quite accurately estimate the number of babies expected to be born given the ages of their mothers[1,2], and as we have a good idea how many babies are actually being born, it is fairly straightforward to calculate how many births are being prevented as a result of prenatal screening.
Here are estimates for the UK, based on live births data from the Office for National Statistics for England and Wales, and from National Records of Scotland and the Northern Ireland Statistics and Research Agency:
This shows the number of live births of children with Down syndrome in each year (green bars) and the 5-year moving average live births of children with Down syndrome (red line). It also shows the additional number of live births we would expect each year had there been no prenatal diagnoses and subsequent terminations (blue bars) and the 5-year moving average for total UK births in the absence of screening (orange line). The reduction rate (the proportion of babies not born as a % of all babies expected to be born), averaged over 5 years, is also provided (yellow dashed line).
|Year||Live births of babies with Down syndrome||5-year UK average births||Additional births in the absence of selective terminations||Total births in the absence of selective terminations||5-year UK average births in the absence of selective terminations||5-yr UK average reduction rate (%)|
- Between 1991 and 2015, an estimated 18,925 fewer babies with Down syndrome were born in the United Kingdom as a result of prenatal screening and diagnosis and subsequent decisions to terminate pregnancies
- In the absence of prenatal screening, we would currently expect an average 1,870 babies to be born each year with Down syndrome in the UK (compared to the 847 actually born)
- The average reduction rate has risen from 38% in the early 1990s to 55% today (the rate at which the expected live birth rate is reduced as a result of screening and pregnancy decisions)
A common question:
- Given registry data suggests that around 23,600 pregnancies diagnosed with Down syndrome were terminated between 1991 and 2015, how could only 18,925 fewer babies have been born? This is because a relatively high number of pregnancies with Down syndrome result in miscarriage or stillbirths. In other words, many of the terminated pregnancies would not have resulted in a live birth if not terminated.
Once again, I must acknowledge helpful discussions about the data with my colleague Gert de Graaf. However, any errors are all mine!
- Morris JK, Mutton DE, Alberman E. (2002) Revised estimates of the maternal age specific live birth prevalence of Down’s syndrome. Journal of Medical Screening, 9, 2-6. http://msc.sagepub.com/content/9/1/2.full.pdf
- Wu J, Morris JK. (2013) Trends in maternal age distribution and the live birth prevalence of Down’s syndrome in England and Wales: 1938–2010. European Journal of Human Genetics, 21, 1016–1019. https://doi.org/10.1038/ejhg.2012.294