Background
The incidence of both sexually transmitted infection (STI) and preterm birth is high among Indigenous women in the Northern Territory, Australia. It was hypothesised that these factors are linked.

Aims
To analyse whether antenatal STI is associated with preterm birth among Northern Territory Indigenous women.

Materials and Methods
A retrospective case–control study was conducted at a tertiary maternity hospital in the Northern Territory. Rates of STI among pregnant Indigenous women were compared between cases (singleton births at <37 weeks gestation) and controls (singleton births at 37 or greater weeks gestation). The association between the composite of any STI (chlamydia, gonorrhoea, trichomonas or syphilis) and preterm birth was evaluated by logistic regression analysis, adjusting for confounders. Secondary endpoints were the associations between each of these infections and preterm birth. Results There were 380 cases and 380 controls. Diagnosis of any sexually transmitted infection (composite) in pregnancy was not associated with preterm birth (adjusted odds ratio (aOR) 0.9, 95%CI 0.58–1.39). Women were at increased risk of preterm birth if they had gonorrhoea in pregnancy (aOR 2.92, 95%CI 1.07–7.97); there was no association with chlamydia (aOR 1.38, 95%CI 0.63–3.04) or trichomonas (aOR 0.66, 95%CI 0.39–1.12). There were three syphilis diagnoses among controls and none among cases. Conclusions Sexually transmitted infection (considered overall) in pregnancy did not affect preterm birth risk among Northern Territory Indigenous women. An association with preterm birth was observed for gonorrhoea in pregnancy but not with chlamydia, trichomonas or syphilis.