Worldwide, Chlamydia trachomatis (CT) is the most common sexually transmitted bacteria. The improved understanding of CT pathophysiology in recent years became possible through DNA amplification technique and genome cloning. This paper updates informations on chlamydial infection in pregnant women, its pathophysiology, diagnostic methods, prevention and treatment. There is increasing evidence that Chlamydia trachomatis infection may result in a number of adverse pregnancy outcomes, including early and late abortion, infection of the foetus, stillbirth, premature rupture of membranes, prematurity and postpartum endometritis. Ectopic pregnancy is often associated with a previous tubal chlamydial infection. C. trachomatis infection in newborns may be acquired during pregnancy or during vaginal delivery, and it may result in neonatal conjunctivitis and/or pneumonia. We discuss benefits of early treatment of chlamydial infections in pregnant women and present guidelines for treatment. Screening should lead to early detection and treatment of men and women with chlamydial infection and thereby reduce the incidence of pelvic inflammatory disease, tubal infertility and ectopic pregnancy.