BACKGROUND Vaginal bleeding is a common complication during pregnancy, which is observed in about 1/4 of pregnancies and in half of cases can lead to abortion. If vaginal bleeding happens during pregnancy some adverse pregnancy outcomes, including perinatal mortality and morbidity, low birth weight and preterm delivery will be increased. OBJECTIVE The aim of this study was to determine the relationship between vaginal bleeding and its characteristics in the first and second trimester of pregnancy and preterm labor. METHODS This is a case-control study conducted on 440 pregnant women referred to Al-Zahra Hospital in Rasht, Iran. Data were collected by a form. The form included demographic characteristics and confounding factors, the occurrence of bleeding during pregnancy and its features. Data were analyzed by T test, chi square and logistic regression in SPSS 16. RESULTS Findings showed that vaginal bleeding was associated with 3 times increased risk of preterm delivery (OR: 3, 1.84-4.89). Also, findings showed that bleeding characteristics including bleeding time, frequency, severity and intensity was significantly associated with preterm labor. CONCLUSIONS According to significant association between vaginal bleeding and preterm delivery, it seems that performing some interventions to prevent preterm labor could be appropriate.