The older a woman gets the more likely she is to suffer complications during pregnancy.

It is estimated that approximately 12,000 women a year become mothers during their forties.

There is no reason why a fit and healthy woman in her forties should not have a successful pregnancy.

The key to minimising risk is to adopt a healthy lifestyle. A good diet and regular exercise is important, and, as with all pregnant women, smoking is an extremely bad idea.

"Fewer than 600 women a year give birth over the age of 45"

Regular rest and a good night's sleep are also recommended as is regular stretching exercises to maintain circulation in the pelvic area. This prevents the pooling of blood in the placenta.

All women who plan to get pregnant should take daily doses of folic acid to reduce the risk of foetal abnormalities.

It is also vital - and standard medical practice - for older women have regular scans during their pregnancy so that doctors can keep a close eye on the development of the foetus.

Tests such as amniocentesis - the withdrawal of amniotic fluid from around the foetus - and chorionic villus sampling are also available to detect abnormalities.

BBC doctor Rosemary Leonard said: "There is a myth that 40-something women are not fit to have children, in fact they are very fit to have children.

"There are inevitability risks both to the child and the mother. But by and large most 40-something women sail through their pregnancy."

Down's syndrome

It is known that older women do run a higher risk of having children with genetic disorders, such as Down's syndrome.

There is a one in 2,000 chance of a woman in her 20s having a Down's baby. By the age of 45 the risk has increased to approximately one in 30.

Older women are also more at risk of a condition known as aneuploidy, in which individual chromosomes, the structures that carry the genetic information, are lost, causing defects in the baby.

Normally the loss of a chromosome leads to a spontaneous abortion at an early stage of pregnancy.

An exception is the loss of an X chromosome that produces Turner syndrome.

High blood pressure

There is also a higher risk that the woman will suffer from high blood pressure.

This can lead to the potentially fatal disorder pre-eclampsia which can cause damage to the heart, kidneys and brain.

Age-related problems such as the development of fibroids in the uterus can also be a factor.

Babies do not grow as well in older women, and failure of the placenta, which provides the foetus with essential nutrients taken from the mother's bloodstream, is more common.

Potentially dangerous ectopic pregnancies, where the fertilised egg implants itself outside the womb, are also more common in older women as is diabetes.

An added disadvantage for Mrs Blair is that it is 11 years since the birth of her last child. This could make labour difficult, and medical assistance with the delivery more likely.

Obstetricians tend to treat a woman who has such a gap between children as though she is giving birth for the first time.