Although the chance of giving birth to a baby with Down syndrome begins to rise quite quickly for mothers aged over 30 years, in most countries it has traditionally been the case that more babies are born to younger than to older women. This was because, in general, many more babies were born to younger women than to older women. But, is this still the case?
This question arose a few weeks ago when reviewing a draft of an updated information booklet on Down syndrome published by Early Support: are more babies with Down syndrome born to mothers aged under 35 years than to mothers aged 35 years or over?
It is well established that the chance of giving birth to a baby with Down syndrome is higher among older women than it is among younger women. Since 1989, a national registry has recorded all diagnosed cases of Down syndrome (prenatal and postnatal) in England and Wales. This large dataset offers probably the best estimation of the chances of giving birth to a baby with Down syndrome for mothers of different ages. From around 30 years of age (at the time of birth) the chance increases markedly, rising from 1 in 1,339 at 25 years to 1 in 352 at 35 years and 1 in 35 at 45 years. This graph shows how many babies would be born with Down syndrome in the absence of prenatal screening and selective terminations to mothers of different ages (per 1,000 livebirths):
Estimated prevalence of babies with Down syndrome by age of mother at birth (per 1,000 livebirths). Source: http://dx.doi.org/10.1136/jms.9.1.2
Clearly, of babies born to mothers aged 35 years or older, more will have Down syndrome than among babies born to mothers aged under 35 years. We might suppose, therefore, that more babies with Down syndrome are born to older mothers. In the past, this has not been true, because many more babies were born to younger mothers than to older mothers. However, the relative difference has narrowed over the past 20 years. As a paper published in Down Syndrome Research and Practice a few years ago pointed out, the percentage of all births in England and Wales to mothers aged 35 and over increased from 9% in 1989 to 19% in 2003. During this time, the proportion of children with Down syndrome born to mothers aged 35 and over rose from 24.2% to 50.9%.
So, what has happened since 2003? Here is the latest data –
|Year||Mothers aged <35 years||Mothers aged ≥35 years||Mothers age unknown|
Assuming that the outcomes of pregnancies for which outcomes are unknown and for those for which maternal ages are unknown were similar to the known outcomes, then we can conclude that in 2009 and 2010 55% of babies with Down syndrome were born to ‘older’ mothers:
Proportion of babies born with Down syndrome in England and Wales born to mothers aged <35 years and mothers aged ≥35 years, 1989-2010. Source: National Down Syndrome Cytogenetic Register (NDSCR).
The trends are likely to be quite similar in other developed countries.