The number of women in England and Wales conceiving babies with Down's Syndrome has leapt by more than 70 percent in the last two decades, researchers said on Tuesday.
The figures appear to have risen as a direct result of the rising numbers of women who fall pregnant in their 30s and 40s.
The risk of conceiving a baby with Down's increases with age. For a woman aged 30, the chance is one in every 940. By age 40, the risk rises to one in 85.
Experts at the University of London, analysed data from the Down's register dating back 20 years and found that in 1989-90, some 1,075 women were diagnosed as carrying a Down's baby. By 2007-08, that number had risen to 1,843, a 71 percent increase.
Improved prenatal screening, and the numbers of women being scanned, has also played a role, with more Down's pregnancies being picked up.
"What we're seeing here is a steep rise in pregnancies with Down's Syndrome but that is being offset by improvements in screening," said Joan Morris, professor of medical statistics at Queen Mary college, who led the study.
"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."
The researchers noted a sharp increase in the proportion of Down?s Syndrome pregnancies in women under the age of 37 being detected by screening, rising from three percent to 43 percent, while the figure for women over 37 remained constant at around 70 percent.
The findings also show the number of babies born with the condition has remained stable over the same period, falling from an annual total of 752 to 743.
Only eight percent of couples diagnosed with a Down?s Syndrome pregnancy decide to keep the baby. Some 92 percent chose to undergo an abortion, a statistic which has remained constant.
Results of the study, which examined cases in England and Wales, were published in the British Medical Journal.