Objectives Chlamydia trachomatis colonisation is common in pregnant women, and it has been claimed that mother-to-child transmission may occur in 10%–70% of deliveries. C. trachomatis infections are nevertheless rarely encountered in infants in clinical practice. In order to evaluate the reason for this discrepancy, we designed a nationwide study of the C. trachomatis vertical transmission.

Methods Children with a possible C. trachomatis infection were identified from two national health registries in 1996–2011. Copies of the children's medical records were reviewed and maternal serum bank samples obtained during the index pregnancies were analysed for C. trachomatis antibodies. The risk of vertical transmission was calculated using data from two earlier studies in which nucleic acid amplification test (NAAT) positivity and seroconversion rates among women in the general population were reported.

Results Altogether 206 children had a possible C. trachomatis infection, which represents 0.22 per 1000 live births (95% CI 0.19 to 0.25). The risk of vertical transmission among the estimated 24 901 NAAT-positive mothers was 0.8% (95% CI 0.7 to 0.9). Based on the annual seroconversion rate of maternal antitrachomatis antibodies, the risk of vertical transmission was 1.8% (95% CI 1.5 to 2.0). Altogether 35% of the maternal serum samples obtained in the first trimester of a pregnancy leading to a C. trachomatis infection in the infant were negative, implying that the infection was acquired during pregnancy.

Conclusions C. trachomatis infections in infants were rare, with a population-based occurrence of 0.22 per 1000 live births. The risk of vertical transmission of C. trachomatis in the population was <2%, which is significantly lower than reported earlier.