Successful treatment for gum disease cuts the risk of pregnant women giving birth early, US research suggests.

The preliminary research showed those whose gum disease was not treated successfully were three times more likely to give birth before 35 weeks.

The study of 160 women was presented to the annual conference of the American Association for Dental Research.

UK experts said the finding was "controversial" but advised pregnant women to take care of teeth and gums.

Doctors have previously established that severe gum infections cause an increase in the production of prostaglandin and tumour necrosis factor, chemicals which induce labour, to be produced.

The study by researchers at the University of Pennsylvania enrolled women who were between six and 20 weeks' pregnant.

All of the volunteers had gum disease. These women were given treatment, which was successful in one third of the cases.

The researchers found a "strong and significant association" between successful treatment and full-term births.

Those whose treatment did not work were "significantly more likely" to give birth before 35 weeks.

'Controversial area'

UK experts warned that this was a small study and further research was needed.

Professor Iain Chapple, from Birmingham Dental School, said this was a "controversial area", and that while some previous studies had shown an association between gum disease and early births, others had shown no association.

He said the results "could reflect behavioural differences in the successfully treated group versus the unsuccessfully treated group" such as "poorer diets, smoking status, alcohol intake and many other issues".

But Dr Nigel Carter, chief executive of the British Dental Health Foundation, said: "This paper adds to the growing evidence around links between gum disease and pre-term babies.

"It is further strong evidence that pregnant women should take care of their periodontal health and receive appropriate treatment during their pregnancy to reduce as far as possible their chance of a pre-term birth."

'Only one clue'

This advice was echoed by the pregnancy research charity Tommy's.

Its consultant midwife, Annette Briley, said: "Women in the UK do get free dental care during pregnancy and for a year after the baby's birth.

"It is therefore good to go to the dentist early in pregnancy and ensure that your mouth, teeth and gums are as healthy as they can be.

"However, the causes of preterm birth are multifactorial and many women with no periodontal disease may still have their babies early, this is only one clue to one cause."