Genital Chlamydia trachomatis infection is the commonest bacterial sexually transmitted infection worldwide. Infection prevalence peaks in young women aged between 18-25 years. Infection in women has been associated with reproductive tract pathology, infertility, and adverse pregnancy outcomes including miscarriage, early membrane rupture, pre-term labor, and postpartum endometritis. However, the evidence base varies with the population studied and the methods used to detect infection. There may be differential consequences for pathology associated with primary or recurrent infection during pregnancy. These differences may be potentiated by physiological differences in the host response to infection in the pregnant state. Such changes have particular relevance for infections of the reproductive tract. Cost effectiveness estimates for screening during pregnancy require basic knowledge of the natural history of infection and the host response to calculate associated risks. Our level of knowledge is hampered by the lack of good experimental models for human pregnancy. To make rational decisions about screening of pregnant women there is a need for case control studies that compare detection of infection by nucleic acid amplification tests with evaluation of immunity to the infection.