A paper published recently in The Journal of Pediatrics estimates the population of people with Down syndrome alive in the USA in 2008 – not the oft-stated 400,000. Even this may be an overestimate.
It is important to understand the size and the demographics of populations of people with Down syndrome in order to plan services. In an important paper, recently published online by The Journal of Pediatrics, Angela Presson and colleagues present an estimate of the current population of people with Down syndrome living in the USA in 2008, which they calculate to be 250,700 with a 90% uncertainty interval of 185,900-321,700.
This is substantially lower than the 400,000 figure that is often given as the number of Americans living with Down syndrome. It is not clear what this traditional figure is based on, but it seems to confuse birth prevalence with the population (of all ages) prevalence. Because the proportion of babies born with Down syndrome varies over time and because people with Down syndrome do not live as long as other people and longevity trends have been changing over time, birth prevalence and population prevalence are quite different. For example, a study using data from 10 population registries in the US reported a live birth prevalence rate of Down syndrome of 11.8 per 10,000 live births in 2003. If applicable nationally and assumed to be the population prevalence, and given a 2003 population of 290,107,933, then we could estimate a population of 342,327. Still not 400,000, though.
Furthermore, the same 2009 paper reports the population prevalence of Down syndrome among children and adolescents aged 0 to 19 years in 2002 in 10 US regions to be 10.3 per 10,000. Using this as the prevalence rate for the population of all ages in 2003 would give a lower estimate of 298,811. Of course, the prevalence rate Down syndrome among people aged over 19 years will actually be lower, and therefore this is most likely an overestimate.
It could be argued that recent birth and youthful population prevalence rates underestimate historic prevalence rates because of the availability of prenatal diagnosis and selective terminations since the 1970s. However, at the same time many more babies have been conceived as parents choose to have children later in life. Moreover, improved healthcare as markedly improved survival rates for children with Down syndrome over the past 30 years – particularly those with cardiac defects.
There is no national birth registry in the US, and no census data and few studies of the population of people with Down syndrome. So, how did the authors of the study arrive at their estimate for the US population of people with Down syndrome? Put simply, the authors (a) estimated the number of births and (b) estimated the ages at which those born died – and therefore those who were still alive.
The study authors estimated births using (a1) data on the maternal-age-specific chances of giving birth to a baby with Down syndrome combined with (a2) data on all births of babies and the ages of their mothers and (a3) an adjustment for the impact of prenatal diagnosis and subsequent selective terminations. The authors estimated age of death by looking at the average age of death of people with Down syndrome where recorded in death certificate data from 2006 and 2007.
The maternal-age-specific chances of giving birth to a baby with Down syndrome (a1) are known with reasonable certainty and large and reliable data sets on births are maintained (a2) in the US. We can therefore quite accurately calculate the number of babies who would be expected to be born in a given year in the absence of selective terminations. For example, using these methods it is possible to calculate around 350,000 babies would probably have been born in the US in the past 60 years in the absence of selective terminations.
Unfortunately, there is limited data on the impact of prenatal diagnosis policies and technologies since the 1970s in the US. The authors of this study use an adjustment factor based on data reported putting the ratio of live births to live births + terminations at 0.874. They apply this adjustment to their estimates of births in each year from 1980 to 2007.
There are some potential problems with this estimate of the impact of selective terminations on live births. Firstly, the authors assume that the ratio of live births to live births + terminations remained constant during this period. Two factors influence this ratio: the proportion of pregnancies diagnosed prenatally and the choices of expectant parents when receiving the diagnosis. Unfortunately, the paper the authors cite in support of the constancy of this ratio only looks at termination rates – in other words the rate at which expectant parents choose to terminate given a diagnosis. This averages at around 91% (following the decision to have an invasive diagnostic test) and, as far as we can tell, has not fluctuated much over the past 20 years. However, during this time, the availability of prenatal screening tests has changed substantially, with their widespread offer to women of all ages being relatively recent. In England and Wales (where we have a national registry), for example, the ratio of live births to live births + terminations went from 0.71 in 1989 to 0.40 in 2011.
Secondly, the number of terminations is not the same thing as the number of births prevented. This is because quite a high proportion of pregnancies with Down syndrome are lost. In other words, many of the pregnancies terminated would not have resulted in a live birth. For example, in England and Wales, the recent ratio of live births to live births + terminations of 0.40 suggests a live birth reduction rate of 60%. It is, in fact, closer to 50%.
Thirdly, the rate seems very low. The authors assume live births are 12.6% lower from 1980 to 2007 than those expected in the absence of selective terminations. I previously mentioned that a study using data from 10 population registries in the US reported a live birth prevalence rate of Down syndrome of 11.8 per 10,000 live births in 2003. The authors estimate that births in that year would have been around 7,000 in the absence of selective terminations. If correct, this would have been a live birth prevalence of 17.1 per 10,000. If this were the case, the rate of reduction of births resulting from selective terminations would have been 31.0% – not 12.6%. The live births at least for part of the period from 1980 to 2007 may therefore be overestimated.
It should be noted that estimates of population prevalence derived from births and survival estimates do not account for the effects of immigration and emigration. (Migration effects may also distort population samples.) There is (and has been) net immigration in the US and it is therefore possible that such estimates fail to account for immigrants with Down syndrome. However, it seems unlikely that there are (or has been) a high number of immigrants with Down syndrome, nor that the prevalence of people with Down syndrome among immigrants in general is disproportionately high. Indeed, the opposite may be more probable
In conclusion, this study makes it clear that the suggestion that there are 400,000 Americans with Down syndrome is not credible. It is most likely that the number is closer to 250,000 – perhaps as high as 300,000 – perhaps substantially lower than 250,000.