Are more babies with Down syndrome born to younger mothers?

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)

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
Live born Outcome
unknown
Live born Outcome
unknown
Live born Outcome
unknown
1989 536 0 172 8 22 0
1990 521 1 197 11 8 0
1991 505 2 205 7 15 0
1992 448 5 183 13 24 0
1993 430 1 159 7 23 0
1994 399 9 183 15 43 1
1995 348 5 192 18 28 2
1996 370 4 192 9 33 0
1997 382 5 233 14 38 0
1998 352 6 226 20 42 0
1999 318 10 221 17 52 2
2000 315 23 228 38 35 0
2001 254 17 214 62 94 2
2002 271 31 199 72 111 0
2003 258 18 268 48 85 4
2004 286 17 281 63 90 1
2005 298 36 308 98 127 6
2006 281 43 316 86 152 11
2007 288 38 336 72 79 5
2008 300 27 324 56 110 3
2009 292 28 352 43 126 1
2010 271 26 326 70 122 1

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 younger than 35 years and mothers aged 35 years or older, 1989-2010

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.