11 January 2011 last updated at 06:49 ET Share this page An ectopic pregnancy is when an embryo implants outside the womb in the Fallopian tube
Women who have had chlamydia are at greater risk of an ectopic pregnancy because of a lasting effect of the infection, according to new research.
A study by Edinburgh University experts has shown for the first time of how the sexually-transmitted infection can increase the risk of ectopic pregnancy.
An ectopic pregnancy is when an embryo implants in the Fallopian tube.
Women who have had chlamydia are more likely to produce a protein in their Fallopian tubes, the study said.
Increased production of this protein, known as PROKR2, makes a pregnancy more likely to implant outside of the womb.
Sexually transmitted
The study follows on from research, also at Edinburgh University, 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.
The 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 Edinburgh University'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."