Women who become infected with sexually transmitted infections (STIs) before or during pregnancy may be at an increased risk of experiencing complications, such as a premature birth or a stillbirth, a new study suggests.

The study looked at the effects of chlamydia and gonorrhea. Chlamydia is the most common STI in the developed world. Usually, it has no obvious symptoms, especially in women, however if left untreated, it can lead to serious complications such as infertility, and ectopic pregnancies.

Gonorrhoea is most commonly spread during sexual intercourse with an infected partner, although it can also be passed from mothers to babies during birth. If left untreated, it can lead to pelvic inflammatory disease, ectopic pregnancies and infertility.

Australian scientists set out to determine whether being infected with either of these STIs in the lead-up to, or during pregnancy, could have any impact on the birth or the baby. They looked at the birth records of over 354,000 women who gave birth for the first time between 1999 and 2008.

Among the women, over 3,600 had been diagnosed with chlamydia at least once before the birth, most of these before the baby was conceived.

Almost 200 were diagnosed with gonorrhea before the birth, again most of these were before the baby was conceived.

The study found that even when other factors were taken into account, women who had been infected with either of these STIs were at an increased risk of suffering pregnancy complications.

For example, women who had been diagnosed with chlamydia were 40% more likely to have a stillborn baby and 17% more likely to have a premature birth.

Meanwhile, those who had been diagnosed with gonorrhea were at least twice as likely to have a premature birth. However, there were not enough women in the study with gonorrhea to assess whether the infection had any impact on the risk of having a stillbirth.

The scientists from the University of New South Wales noted that among women with chlamydia, the risk of a premature birth was the same whether they were diagnosed more than 12 months before conception, within 12 months of conception or during pregnancy.

"Our results suggest that sexually transmissible infections in pregnancy and the pre-conception period may be important in predicting adverse obstetric outcomes," they concluded.

Details of these findings are published in the journal, Sexually Transmitted Infections.

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