This is because more women are having their babies later in life, when there is a greater risk to the foetus.
For women aged 30 there is a one in 940 risk, but by 40 the risk rises to one in 85.
Despite improved screening, the number of Down's pregnancies has gone up to 1,843 in 2008 compared to 1075 diagnoses in 1990.
However, the number of babies being born with Down's syndrome has gone down due to improvements in antenatal diagnoses and an increase in abortions of affected foetuses.Around 60,000 people have Down's syndrome in the UK, a genetic disorder that affects learning and mental development
Joan Morris, professor of medical statistics at Queen Mary who led the research (published in the British Medical Journal), said: "What we're seeing here is a steep rise in pregnancies with Down's syndrome but that is being offset by improvements in screening.
"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 Down's Syndrome Association is keen to educate parents about Down's. Carol Boys from the charity believes that fewer people would abort if they were better informed about the syndrome. She said: "It is... important that families undergoing the screening process are given non-directive counselling and accurate, up-to-date information about Downs' syndrome."