Becoming infected with chlamydia or gonorrhoea in the lead-up to, or during, pregnancy, increases the risk of complications, such as stillbirth or unplanned premature birth, indicates research published online in the journal Sexually Transmitted Infections.

The researchers analysed the birth records of more than 350,000 women who had had their first baby between 1999 and 2008 in New South Wales, Australia's most heavily populated state.

The researchers wanted to find out if infection with either chlamydia or gonorrhoea in the lead-up to, or during, pregnancy, had any impact on the baby or the birth itself, as there is continuing debate about whether these infections can increase the risk of complications.

The women's birth records were linked back to state records about notifiable conditions, such as chlamydia and gonorrhoea.

Among the 354,217 women who had had their first child between 1999 and 2008, 3658 (1%) had had at least one notifiable chlamydia infection before the birth. And most (81%) of these had been diagnosed before the estimated date of conception.

Just 196 (0.6%) had been diagnosed with gonorrhoea before the birth, with most diagnoses (just under 85%) made before the estimated conception date.

Half of those diagnosed with gonorrhoea had also previously been infected with chlamydia.

In all, 4% of the women had an unplanned premature birth; 12% had babies who were small for dates; and 0.6% (2234) of the babies were stillborn.

Factors such as age, social disadvantage, smoking, and underlying conditions, such as diabetes and high blood pressure, can all increase the risk of birth complications, and this was evident among the women studied.

But even after taking account of all these influential factors, women who had had a prior infection with either chlamydia or gonorrhoea were still at heightened risk.

Women who had had chlamydia were not at increased risk of giving birth to a small for dates baby. But they were 17% more likely to have an unplanned premature birth and 40% more likely to have a stillborn baby.

Women who had had gonorrhoea were more than twice as likely to have an unplanned premature birth, but they were not at increased risk of giving birth to a small for dates baby.

There were too few women with a previous diagnosis of gonorrhoea to be able to assess the impact of the infection to stillbirth.

For women previously diagnosed with chlamydia, the risk of an unplanned premature birth did not differ between those diagnosed more than a year before conception, within a year of conception, or during the pregnancy.

The authors caution that their findings don't allow them to prove cause and effect. The infections may simply be a marker for women at high risk of birth complications.

And while there is some evidence to suggest that chronic inflammation―such as would arise particularly with chlamydia infection―can trigger an unplanned premature birth, trials of prophylactic antibiotics given to women during pregnancy, have not lowered this risk.

Nevertheless, the authors conclude: "Our results suggest that sexually transmissible infections in pregnancy and the preconception period may be important in predicting adverse obstetric outcomes."