A chemical in cigarette smoke has been found to cause a reaction which can lead to ectopic pregnancies, according to Edinburgh scientists.
Research from experts at Edinburgh University said Cotinine triggered a reaction which increased a protein in the Fallopian tubes.
They said the protein, called PROKR1, raised the risk of an egg implanting outside the womb.
Details of the study were published in the American Journal of Pathology.
PROKR1 allows pregnancies to implant correctly inside the womb, but its presence in the Fallopian tubes is believed to increase the risks of this happening outside the womb.
The study found that women who smoked and developed an ectopic pregnancy had twice as much PROKR1 in their Fallopian tubes as women who did not smoke and had previously had a healthy pregnancy.
Researchers believe that too much of the protein prevents the muscles in the walls of the Fallopian tubes from contracting, which in turn hinders the transfer of the egg to the womb.
Dr Andrew Horne, of 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 blood stream and affect seemingly unconnected parts of the body like the reproductive tract."
The study, funded by Wellbeing of Women, analysed tissue samples from female smokers and non-smokers, and from women who had previously had ectopic and healthy pregnancies.
Smoking is thought to increase the risk of an ectopic pregnancy by up to four times.
There are more than 30,000 ectopic pregnancies in the UK each year, with the egg implanting in the Fallopian tube in 98% of cases.
This can cause the tube to rupture and lead to internal bleeding and fertility problems in the future.
Around one in 50 pregnancies in the western world is ectopic. The condition is the leading cause of maternal mortality in the first three months of pregnancy.