Rising numbers of women choosing to have children later in life have led to a sharp increase in Down's syndrome pregnancies, research has revealed.
The number of Down's babies conceived has soared by 71% in the last 20 years as more women delay starting a family until their late thirties or early forties, figures out today show.
But improvements in pre-natal screening mean that more of the affected pregnancies are being detected earlier and most now end in terminations.
As a result the number of babies born with Down's syndrome has remained fairly level at around 750 a year, although it has fluctuated from year to year by around 20%.
The findings, published in the British Medical Journal, highlight the risks run by older mothers. The proportion of mothers aged over 35 doubled between 1989 and 2006 from 6% to 15%, and the number of those over 40 is rising even faster.
The risk of a Down's syndrome pregnancy is 16 times greater in a mother over 40 than in one aged 25.
Joan Morris, professor of medical statistics at Queen Mary University of London, said: “There has been an enormous shift in the age of mothers, it really has been dramatic. Down's is the commonest chromosomal abnormality in babies and the commonest abnormality of any kind in babies born to mothers over 45.”
There were 1,075 Down's syndrome pregnancies in 1989-90, rising to 1,843 in 2007-8. On the basis of increases in maternal age, a 48% rise in Down's births would have been expected if none had been terminated. In fact, the number of births fell by 1% because of widespread screening. Professor Morris said: “The proportion of women who had a termination has not changed since 1989, at 92% of those who had an antenatal diagnosis.”
All women are offered screening but the proportion of those over the age of 37 who accept it had remained the same over 20 years at 70%.
Professor Morris said: “Twenty years ago we were picking up 70% of Down's syndrome babies and today we are still picking up 70%. Now we have got the option of non-invasive screening tests (blood samples and ultrasound) but there is still a core group of women who don't want screening.”
Among women under 37 the proportion of Down's pregnancies detected had risen from 3% to 43% over the same period because of better screening methods. Prof Morris said: “As the test improves we may get near 70% of babies detected, but I don't think we will make 100%.” More research was needed into why women refused screening, she added.