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The number of babies diagnosed with Down's syndrome has risen sharply in the last 20 years - largely due to women delaying motherhood, research shows.

Increasing numbers of older mothers is behind a 71% rise in the number of babies with the condition - from 1,075 diagnoses in 1989/90 to 1,843 in 2007/8.

However, the number of babies born with Down's has remained fairly static over the same period due to improved screening and subsequent abortions, the study found.

Live births of Down's babies fell just 1%, from 752 to 743 (or 1.10 to 1.08 per 1,000 births) over the time period.

If screening had not happened, the number of babies born with Down's would have risen 48%.

Experts from Queen Mary, University of London, analysed data from the Down's register for England and Wales for the study, published online in the British Medical Journal (BMJ).

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.

Joan Morris, professor of medical statistics at Queen Mary, who led the study, 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.

"Other researchers here at Queen Mary have made major improvements to the Down's screening test and it's become more widely available over the last 20 years. This means we're picking up more Down's pregnancies."