Scientists have discovered why women who smoke have a higher risk of developing ectopic pregnancies, a new study revealed today.

Ectopic pregnancies occur when the fertilised egg becomes implanted outside the womb, usually in the fallopian tubes.

There are around 30,000 cases in the UK each year.

Researchers found that female smokers who have had an ectopic pregnancy have raised levels of a protein - PROKR1 - in their fallopian tubes.

The increased presence of this protein is thought to hinder the transfer of the egg to the uterus by preventing the muscles in the walls of the fallopian tubes from contracting.

The team at the University of Edinburgh found that a chemical in cigarette smoke called cotinine is behind the harmful levels of PROKR1.

Dr Andrew Horne, from the university's Centre for Reproductive Biology, said: "This research provides scientific evidence so that we can understand why women who smoke are more at risk of ectopic pregnancies and how smoking impacts on reproductive health.

"While it may be easy to understand why inhalation of smoke affects the lungs, this shows that components of cigarette smoke also enter the bloodstream and affect seemingly unconnected parts of the body like the reproductive tract."

Smoking is thought to increase the risk of an ectopic pregnancy by up to four times.

The egg implants itself in the fallopian tube instead of the womb in 98% of cases.

This can cause the tube to rupture and affect a woman's ability to conceive in future.

The study, funded by the Wellbeing of Women charity, analysed tissue samples from female smokers and non-smokers, and from women who had previously had ectopic and healthy pregnancies.

Scientists found that women who smoked and developed an ectopic pregnancy had double the levels of the protein in their fallopian tubes compared with women who did not smoke and had previously had a healthy pregnancy.

The study is published in the latest edition of the American Journal of Pathology.