To examine the association between colonization by two newly classified species of genital ureaplasmas (Ureaplasma parvum and U. urealyticum) in early pregnancy and subsequent late abortion or preterm birth at <34 weeks of gestation, four species of genital mycoplasmas--Mycoplasma genitalium, M. hominis, U. parvum, and U. urealyticum--as well as Chlamydia trachomatis and Neisseria gonorrhoeae were examined by PCR-based methods in a prospective cohort study of 877 women with singleton pregnancies at <11 weeks of gestation. Antibiotics were used only in cases in which C. trachomatis and/or N. gonorrhoeae was detected. Multivariate logistic-regression analysis was used to assess independent risk factors after taking maternal low body weight and past history of preterm birth into account. M. genitalium, M. hominis, U. parvum, U. urealyticum, C. trachomatis, and N. gonorrhoeae were detected in 0.8%, 11.2%, 52.0%, 8.7%, 3.2%, and 0.1% of these 877 women, respectively. Twenty-one (2.4%) women experienced late abortion or preterm birth at <34 weeks of gestation. Three factors-detection of U. parvum in the vagina (odds ratio [OR], 3.0; 95% confidence interval [CI], 1.1 to 8.5); use of antibiotics, such as penicillin and cefatrizine, for incidental inflammatory complications before 22 weeks of gestation (OR, 4.2; 95% CI, 1.6 to 10.0); and past history of preterm birth (OR, 10.4; 95% CI, 2.7 to 40.5)-were independently associated with late abortion and preterm birth. In conclusion, vaginal colonization with U. parvum, but not U. urealyticum, is associated with late abortion or early preterm birth.