The number of women delaying motherhood is the reason for a 71% increase in the number of babies diagnosed with Down's syndrome in the last two decades, according to research published today. There were 1,843 diagnoses of Down's syndrome in England and Wales in 2007-2008 compared with 1,075 in 1989-1990.
The risk of having a baby with Down's syndrome is one in 940 for a woman aged 30. By the age of 40, the risk increases to one in 85.
Despite the increased diagnoses, the number of babies born with Down's syndrome has remained almost static because of improved screening and subsequent abortions, researchers at Queen Mary, University of London, found.
Live births of Down's babies fell from 752 to 743 –or 1.10 to 1.08 per 1,000 births – over the period of time covered by the research.
If screening had not happened, the number of babies born with Down's would have risen by 48%.
"What we're seeing here is a steep rise in pregnancies with Down's syndrome, but that is being offset by improvements in screening," Joan Morris, the professor of medical statistics at Queen Mary, who led the study, said.
"It was thought that these improvements would lead to a decrease in the number of births with Down's syndrome. However, due to increases in maternal age this has not occurred."
Morris said there had been "major improvements to the Down's screening test" which had "become more widely available over the last 20 years".
She said this had contributed in a rise in the proportion of diagnoses among women under the age of 37.
"Among mothers aged 37 years and older, a consistent 70% of affected pregnancies were diagnosed antenatally," the study said.
"In younger mothers, the proportions of pregnancies diagnosed antenatally increased from 3% to 43% owing to improvements in the availability and sensitivity of screening tests."
The proportion of mothers diagnosed with a Down's syndrome pregnancy who decided to terminate has remained constant at 92%.
However, the study showed that the actual number of abortions has risen because more babies are now being diagnosed with Down's.
Because screening is now carried out earlier than ever before in a pregnancy, a proportion of cases would have gone on to miscarry naturally.
"Increases in maternal age would have caused a 48% increase in births with Down's syndrome in the absence of terminations between 1989/91 and 2005/7," the authors concluded.
"However, terminations of Down's syndrome pregnancies due to an increase and improvements in antenatal screening have caused the number of live births with Down's syndrome to remain constant." The study used data from the Down's register for England and Wales, published online by the British Medical Journal.
The anonymous register holds 26,488 antenatal and postnatal diagnoses of Down's syndrome made by all cytogenetic laboratories, which analyse chromosomal abnormalities, since 1989.