Scientists claim there is a greater risk of ectopic pregnancy among women who have had the sexually transmitted infection (STI) chlamydia.
One lasting effect of the infection is that women are more likely to produce a protein known as PROKR2 in the Fallopian tubes, making an embryo more likely to implant outside the womb.
Researchers at the University of Edinburgh found that, for the first time, this shows how chlamydia can raise the risk of ectopic pregnancy, and follows an earlier study which revealed a similar protein can raise the risk of the same complication occurring in smokers.
Chlamydia, the UK's most common sexually transmitted infection, is treatable but sufferers often don't know they have it, as it can be symptom-free. It can lead to scarring in the Fallopian tubes and infertility.
Even more worryingly, the research shows that this disease can cause alterations to the Fallopian tubes in much more subtle ways, without the clear evidence of scarring or blockages.