Q&A: Miscarriage

Following the news that Cherie Blair, the wife of prime minister Tony Blair, is being treated in hospital following a miscarriage, BBC News Online looks at common causes for miscarriage.

What causes it?

There are a number of causes. One common reason is a genetic problem that prevents the baby from developing normally.

In some cases, the foetus does not implant properly into the lining of the womb.

Other causes include hormone imbalances in the mother or problems with a woman's immune system.

In many cases, the cause is unknown.

How common are they?

About one in four pregnancies ends in miscarriage. Sometimes this happens even before a woman knows she is pregnant.

The risk of miscarriage lessens as the pregnancy progresses. It decreases dramatically after the 8th week.

Is miscarriage more common in older women?

According to the Royal College of Obstetricians and Gynaecologists, women who are pregnant at the age of 25-30 have a miscarriage rate of 16%.

By the age of 40, this rises to 25%.

"Then as women get older through the next 10 years the rate goes up and up and up and would probably be at least 50% by 47 or 48," says College spokesman Peter Bowen-Simpkins.

This is because the likelihood of chromosomal abnormalities increases with age.

Do miscarriages need treatment?

Some miscarriages are complete and no surgery is needed.

In other cases, a small operation is carried out to reduce the chance of infection and to stop the bleeding.

This is done under anaesthetic and is known as a D&C (dilatation and curettage).

Are there any complications?

Very rarely, the procedure can lead to an infection of the womb lining.

This is usually treated with a course of antibiotics.