Tuesday October 27 2009
Older mothers are more likely to have children with Down's
Several newspapers have reported on the steep rise in Down’s syndrome pregnancies, said to be due to recent shifts to later motherhood.
These findings come from The National Down Syndrome Cytogenetic Register, which has gathered data on the number of Down’s diagnoses and births since 1989. The register’s data shows that overall diagnosis of the condition has risen by 71% over the past 20 years, from 1,075 in 1989 to 1,843 in 2008.
However, improvements in the screening process has led to a decrease in the number of Down’s syndrome births due to a high rate of terminations. The study also suggests that the rise in Down’s diagnoses is associated with a rise in the number of women putting off having children until later in life.
Where did the story come from?
The study was conducted by Professors Joan Morris and Eva Alberman of Barts and London School of Medicine and Dentistry, and was published in the British Medical Journal. The NHS Fetal Anomaly Screening Programme funded the National Down Syndrome Cytogenetic Register to collect the study data until March 2009.
What kind of scientific study was this?
This was a time trend study looking at the trends in Down’s syndrome conceptions and births in England and Wales from 1989 to 2008. It used data from The National Down Syndrome Cytogenetic Register, which contains details on all babies diagnosed antenatally (before birth) and following birth. The register is believed to have captured around 93% of all births and pregnancy terminations during this period, also recording the majority of mothers’ ages. For the purposes of the register, the women were placed in one of two age ranges: under 37 or 37 and above.
The register’s statistics for Down’s syndrome were compared to data on other congenital conditions and from the UK Office for National Statistics.
What were the results of the study?
The study found that although there were a similar number of births in 1989-90 and 2007-8, antenatal and postnatal diagnoses of Down’s syndrome increased by 71%, from 1,075 in 1989-90 to 1,843 in 2007-8.
Antenatal screening and subsequent terminations of pregnancy resulted in an overall 1% fall in the number of babies born with Down’s syndrome (752 in 1989-90 to 743 in 2007-8). This equates to a decrease from 1.10 babies per 1,000 births to a current rate of 1.08 babies per 1,000 births. The research estimates that if screening had not been in place the actual numbers of live births with Down’s syndrome would have increased by 48%, from 959 to 1,422. This estimated rise appears to be attributable to the fact that couples are starting families at a later stage in life.
The study found that in the older mothers’ group (37 and above), 70% of those who gave birth to babies with Down’s syndrome had the diagnosis made antenatally (before the birth). This proportion was consistent across the two time-periods.
In younger mothers the proportion with antenatal diagnoses was far less, although it did increase from 3% in 1989-90 to 43% in 2007-2008. So although most Down’s syndrome diagnoses in babies of younger mothers are still made after the birth, the number of antenatal diagnoses is increasing.
What interpretations did the researchers draw from these results?
The researchers say that the number of live births in 1989-1990 was similar to that in 2007-2008, but there has been a 71% increase in the rate of antenatal and postnatal Down’s syndrome diagnoses between those times. They say that improvements and increased availability of antenatal screening (and subsequent termination in confirmed cases) has offset an increase in Down’s syndrome births resulting from maternal age becoming older.
Additionally, the proportion of antenatal diagnoses has increased most significantly in younger women, as opposed to the relatively constant rate seen in older women.
They also conclude that the number of antenatal diagnoses may increase further, as more women choose screening, and that monitoring the numbers of babies born with Down’s syndrome is necessary to ensure adequate provision for their needs.
What does the NHS Knowledge Service make of this study?
These reliable findings from The National Down Syndrome Cytogenetic Register have observed that the number of Down’s diagnoses made has risen by 71% in the past 20 years, from 1,075 in 1989-90 to 1,843 in 2007-8. Despite the large rise in Down’s pregnancies the increased uptake of screening has led to a decrease in the number of babies born with Down’s syndrome, due to terminations of pregnancy.
The study highlights the important role that screening and testing can have in providing personalised information to parents so that they can make their own decisions how to proceed, whether it be to keep the baby, give it up for adoption or terminate the pregnancy.
The lower proportion of younger mothers (defined by the study as under 37) that get an antenatal diagnosis is likely to be due to a lower uptake of screening in these age groups, who may not choose to undergo screening as they are at a lower risk than older women. But the rise in antenatal diagnoses in younger women over the past two decades shows they have better access to these screening tests.
The study also indicates that the rise in Down’s diagnoses is linked to older maternal age. There are more risks associated with pregnancies in older women, but the decision about when to have children can involve many personal and professional considerations. Some parents simply do not feel ready to have children until they are older.
The trend for later motherhood has resulted in a higher risk of having children with Down’s, with the research estimating that the risk for a 40-year-old mother is 16 times greater than that for a 25-year-old mother.
The option to choose Down’s screening is an important one, regardless of age. While there are harms and benefits of Down’s screening (and further diagnostic testing if Down’s is indicated as a possibility) that must be considered, such tests can offer invaluable information and support to help make the highly personal issues around proceeding with a pregnancy, considering a termination, or for planning for the welfare of a child with Down’s syndrome.
As the researchers conclude, monitoring the numbers of babies born with Down’s syndrome is essential to ensure adequate provision for the needs of both the families, the baby and the child as they grow up.