Chlamydiae are bacteria that possess both RNA and DNA but are incapable of producing adenosine triphosphate outside of cells. Therefore, these organisms are obligate intracellular pathogens that require tissue culture cells for growth in the laboratory. Of the two species, Chlamydia psittaci and Chlamydia trachomatis, only the latter is a genital pathogen associated with neonatal infection. C. trachomatis has 18 serologic variants (serovars) divided into two biologic groups (biovars): oculogenital serovars A to K and lymphogranuloma serovars L-1 to L-3. Oculogenital serovars include those that cause trachoma (serovars A to C), a hyperendemic blinding chronic follicular keratoconjunctivitis that is rare in the United States, while genital and neonatal infections are caused by serovars B and D through K.

The rate of cervical colonization with C. trachomatis during pregnancy varies from 2% to 37%. The highest rates are found in young, unmarried, nonwhite women of lower socioeconomic status. Chlamydial infection during pregnancy is usually asymptomatic, although urethritis and mucopurulent cervicitis can occur. Moreover, chlamydial infection can result
in pelvic inflammatory disease, ectopic pregnancy, and infertility. Pregnant women with cervical chlamydial infection who have IgM antibody against C. trachomatis, however, may be at increased risk for premature rupture of amniotic membranes and delivery of low-birth-weight infants.