Women who have had chlamydia are at greater risk of an ectopic pregnancy because of a lasting effect of the infection.
A new study provides evidence for the first time of how chlamydia can increase the risk of an ectopic pregnancy which occurs when an embryo implants outside the womb, in the Fallopian tube.
Researchers at the University of Edinburgh found that women who had had the sexually transmitted infection were more likely to produce a particular protein in their Fallopian tubes.
Increased production of this protein known as PROKR2 makes a pregnancy more likely to implant in the Fallopian tube.
The study follows on from research, also at the University of Edinburgh, which showed that production of a similar protein increased the likelihood of smokers having an ectopic pregnancy.
Chlamydia is the most common sexually transmitted infection in the UK. It can be treated but often goes undiagnosed because it can occur without symptoms.
The infection is known to cause infertility as it can lead to scarring and blockages in the Fallopian tube.
This research shows, however, that chlamydial infection linked to ectopic pregnancy causes much more subtle changes in the Fallopian tube, without evidence of severe scarring.
The study, published in the American Journal of Pathology, was funded by the Wellbeing of Women and the Medical Research Council.
Dr Andrew Horne, of the University of Edinburgh's Centre for Reproductive Biology, said: "We know that chlamydia is a major risk factor for ectopic pregnancy but until now we were unsure how the infection led to implantation of a pregnancy in the Fallopian tube. We hope that this new information allows health care providers to give women accurate information about risks following chlamydial infection and to support public health messages about the importance of safer sex and chlamydia testing."