Trichomonas vaginalis is a very common, sexually transmitted, infection that may sometimes be asymptomatic or cause vaginitis and urethritis. Recently, it has been associated with adverse obstetric outcomes such as preterm delivery, low birth weight and premature rupture of membranes. Trichomonas vaginalis can be vertically transmitted at birth. It has been found in pharynx and low respiratory tract of neonates with respiratory disease. It has also been involved in some cases of intellectual disability. The recommended treatment is a 2g metronidazole oral single dose, even for asymptomatic patients. This treatment is effective against Trichomonas and its use is safe during pregnancy. We report here a case of Trichomonas vaginalis infection diagnosed during pregnancy in a patient with severe preterm labor. The patient being allergic to nitroimidazole antibiotics, she did not receive any treatment. She finally gave birth at 34 weeks of gestation (WG) and 5 days, with no other adverse outcome than small prematurity.