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Background: Preterm delivery is still a main cause of perinatal mortality and morbidity. This study was designed to find any association between maternal group B streptococcus (GBS) colonization and preterm delivery. Methods: This case-control study was conducted from March 2010 to May 2011 at two referral university hospitals (Alzahra and Shahid Beheshti Hospitals), Isfahan University of Medical Sciences, Isfahan, Iran. The study population included a control group of 200 women with term pregnancy and a case group of 200 women with preterm labor (between 20 to 37 weeks of gestation). Maternal GBS colonization was compared between the two groups. Findings: Maternal age (26.03± 4.81 vs. 27.19± 5.60; P = 0.07) and number of previous pregnancies (1.75± 0.88 vs. 1.83± 1.08; P = 0.52) were similar in the case and control groups. Level of education and gestational age in the control were significantly higher than the case group (P = 0.01 and P < 0.01, respectively). History of antibiotic use during pregnancy was more frequent in the case group compared with the control group (P < 0.01). There were 59/151 (39.1%) and 36/178 (20.2%) positive vaginal samples in the case and control groups, respectively (P < 0.001). There were 55/151 (36.4%) and 30/178 (16.9%) positive rectal samples in the case and control groups, respectively (P < 0.001). Conclusion: This study showed that GBS colonization was statistically higher in preterm labors compared with term deliveries. To understand the mechanism of GBS effect on pregnancy, large observational studies with clearly defined outcomes and prognostic risk factors for preterm delivery are needed. [ABSTRACT FROM AUTHOR]