Identifying women at risk early is important

Professor Kypros Nicolaides, King's College Hospital, London

But UK doctors say there is little they can do to help the women even if they have been identified.

Ultrasounds

For the research, 183 women who had a history of giving birth prematurely were studied by doctors from the University of Alabama.

They used cervical ultrasounds - a scanning technique which uses high frequency sound waves - to measure the cervix.

They also looked for "funnelling" - a change to the shape of the cervix - and "dynamic" shortening which may occur either spontaneously, or after pressure.

About 10% of pregnancies end in spontaneous premature birth before 37 weeks.

But the study found that women whose cervix measured less than 25mm - it is normally about 38mm in length - had a 3.3 times greater risk.

Those who also had dynamic shortening had a 4.5 times increased risk.

In women who had funnelling, the scientists said it was the length of the cervix rather than the funnel shape which led to premature birth.

'Important step'

The cervix comprises the lower part and neck of the uterus.

In pregnancy, it lengthens and becomes a barrier to help retain the foetus.

Near to the end of pregnancy, it shortens in preparation for labour and delivery.

Some women have an incompetent cervix. This means it is too weak to stay closed during pregnancy.

Kypros Nicolaides, professor of fetal medicine at King's College Hospital, London, told BBC News Online: "Identifying women at risk early is important, and this research is an important step forward.

"But frustratingly, it is not possible at the moment to do something about it."

John Grounds, of the medical research charity Action Research, which is funding studies of the subject, said: 'Having a baby too early can be devastating.

"The baby misses out on vital development time in the mother's womb and is at risk of having a myriad of long term physical and mental problems. Others die because we don't have the knowledge to save them.

"Identifying women at risk of premature labour and reducing the likelihood of it happening will be a major milestone in medical research of the 21st century."