Introduction Chlamydia trachomatis (CT) is a sexually transmitted infection having repercussions on reproductive health and impact on the fetus.

Purpose To estimate Chlamydia trachomatis prevalence and risk factors in pregnant women aged 15 to 24 in Brazil.

Methods A national cross-sectional study among pregnant women attending Brazilian public maternity units in 2009. The participants were screened for CT and Neisseria gonorrhoeae, using polymerase chain reaction in urine, and also answered a questionnaire including demographic, behavioural and clinical data.

Results A total of 2071 (86.3%) of 2400 pregnant women selected took part in the study. Their mean age was 20.2 years (SD 2.7). Chlamydia and Gonococcus infection prevalence was, respectively, 9.8% (95% CI 8.5 to 11.1) and 1.0% (95% CI 0.6% to 1.4%). Four per cent of women infected with Chlamydia also had simultaneous Gonococcus infection. CT associated factors were being aged between 15 and 19 [OR=1.6 (95% CI 1.15 to 2.17)], first sex intercourse before 15 years of age [OR=1.4 (95% CI 1.04 to 6.24)], having had more than one sex partner in their lives [OR=1.6 (95% CI 1.13 to 2.26)], having undergone oncotic cytology more than 1 year ago [OR=1.5 (95% CI 1.08 to 2.05)], and having had gonococcal infection [OR=7.6 (95% CI 3.05 to 19.08)].

Conclusions Health programmes need to pay attention to the need to screen for easily curable sexually transmitted infections, such as Chlamydia trachomatis, in populations that are more vulnerable and at greater risk. This study suggests that CT diagnosis should be included as part of the antenatal routine of young pregnant women, since infection prevalence found in this group was high.