ObjectiveSTIs during pregnancy increase adverse pregnancy and birth outcomes and may increase HIV risk. STI syndromic management is standard of care in South Africa. Our study evaluated the prevalence and incidence of STIs in pregnant women and the associated risk factors.MethodsWe combined data from two prospective observational studies of pregnant women enrolled while attending their first antenatal clinic (ANC) visit in Tshwane District and Cape Town. Women ≥18 years were tested at first ANC visit and at their first postpartum visit for Chlamydia trachomatis, Neisseria gonorrhoeae and Trichomonas vaginalis using Xpert assays (Cepheid, USA). We evaluated the prevalence and incidence of STI and the associated risk factors using multivariable regression models.ResultsWe enrolled 669 pregnant women, 64% (n=427) from Tshwane District and 36% (n=242) from Cape Town; 80% (n=534) were women living with HIV (WLHIV) and 20% (n=135) without HIV. At enrolment, 37% (n=250) were diagnosed with at least one STI, of which 76% (n=190) were asymptomatic. STI prevalence was 40% (n=213) in WLHIV and 27% (n=37) in women without HIV (p=0.01). Baseline STI infection was associated with younger age (OR=0.95 per year, 95% CI 0.92 to 0.98), higher gestational age (adjusted OR (aOR)=1.03 per week, 95% CI 1.00 to 1.05), single relationship status (aOR=1.53, 95% CI 1.09 to 2.15) and HIV status (aOR=1.86, 95% CI 1.17 to 2.95). Of 419 participants with no STI at baseline, 21 had an incident STI during follow-up, with a mean follow-up time of 140 days. The incidence rate of STI during pregnancy and early post partum was 15 infections per 100 women-years (95% CI 9 to 23). Younger age was associated with STI incidence.ConclusionOur study shows high prevalence and incidence of STIs in pregnancy, especially in WLHIV, demonstrating the need for STI screening in ANC to prevent adverse pregnancy and birth outcomes. Most STI cases were asymptomatic and would have gone untreated with syndromic management. Aetiological STI screening is urgently needed to reduce the burden of STIs in pregnancy.