Earlier this month, it was revealed the average age at which women in Britain give birth has topped 30 for the first time.

Social scientists put the trend down to a growing need for women to make a career before starting a family and increasing confidence that medical science has cut the risks.

Risks

A spokeswoman for the Family Planning Association said: "Any risks there are in pregnancy will be increased. The health professionals will be much more aware of the risks and be looking out for signs."

Those risks include high blood pressure, pre-eclampsia and a heightened chance of Down's Syndrome.

But she said: "If you have an extremely fit 40-plus woman, the likelihood of having a trouble-free pregnancy is higher than an unfit 20-year-old."

Jerry Hall - model mum - with Gordon Brown

She cited Jerry Hall as an example of a very fit woman in her 40s

Mick Jagger's former lover, who was at Downing Street on Thursday to help launch a charity appeal, was 42 when she gave birth to her fourth child last year.

Men Behaving Badly star Caroline Quentin became a mother for the first time last year at the age of 39.

Actress Emma Thompson, 40, is due to give birth in a couple of weeks.

City high-flier Nicola Horlick gave birth to her sixth child in September at the age of 39.

Emma Thompson - not long to go now

Hollywood star Susan Sarandon was 45 when she gave birth to her third child in 1992.

Sam Dimitry, a consultant obstetrician and gynaecologist who has conducted research into late pregnancies, said Mrs Blair should have more frequent scans in view of her age.

Mr Dimitry, who works at Wexham Park Hospital, near Slough, Berkshire, said: "Some babies do not grow as well in older women and things like placenta failure is more common. She must have regular scans to check for this."