There is little available information on the demographic and behavioral factors associated with Trichomonas vaginalis in pregnant women. Among 13,816 women from six urban clinic centers, the prevalence rate by culture at mid-pregnancy was 12.6%. Women colonized with T vaginalis were significantly more likely to be black, cigarette smokers, unmarried, and less educated (all P less than .01). Several behavioral factors associated with T vaginalis included greater numbers of sexual partners both lifetime and in the last year, 5 years or more of sexual activity, and a history of gonorrhea (all P less than .01). Trichomonas vaginalis-colonized women were less sexually active in the preceding month compared with uncolonized women (P less than .01). Women using either barrier or oral contraception in the 6 months before becoming pregnant were far less likely to be colonized (P less than .01). Other factors such as age, gravidity, income level, age at first coitus, and use of antibiotics, alcohol, or douche during pregnancy were not independently associated with T vaginalis colonization. Because many of the factors predictive of increased risk of colonization have also been shown to be associated with adverse pregnancy outcome, they should be considered in assessing the association of T vaginalis with adverse pregnancy outcome.