Objective: Despite numerous studies assessing the risk of preterm birth in obese women compared with normal weight patients, the scope of the association remains unclear. Therefore, we conducted a systematic overview and meta-analysis to summarize the available evidence from cohort studies on the relationship between maternal obesity and the risk of preterm birth. Methods: We searched bibliographic database: MEDLINE (PubMed) and EMBASE for all relevant articles, with no language restrictions, which were published from January 1990 to September 2010, using a combination of the following search terms: maternal obesity or high body mass index (BMI) and preterm birth or pregnancy outcome. Crude odds ratios (OR) for individual outcomes were calculated for each study and were pooled by using the random-effects model. Results: Twenty seven studies proved eligible under the selection criteria. The crude OR of preterm birth were 1.18 [95% confidence interval (CI) 1.07 to 1.30], 1.28 (95% CI 0.93 to 1.75) and 1.11 (95% CI 0.98 to 1.25) among obese, severely obese and morbidly obese women, respectively, compared with normal weight women. We found an elevated risk of induced preterm birth among obese (OR 1.72, 95% CI 1.45 to 2.04), severely obese (OR 1.70, 95% CI 1.32 to 2.18) and morbidly obese pregnants (OR 1.66, 95% CI 1.33 to 2.07) but a reduction in the risk of spontaneous preterm birth: OR = 0.98 (95% CI 0.80 to 1.20), OR=0.85 (95% CI 0.55 to 1.31) and OR=0.77 (95% CI 0.60 to 0.99), respectively. Obese women, independently of category the of obesity, also are at a higher risk of very preterm delivery (~ <32 weeks). Conclusions: Maternal obesity is associated with an increased risk of medically indicated preterm birth and with a decrease in the risk of spontaneous preterm birth. It appears that slightly increased risk of preterm birth overall may be masked by the increase in the incidence of induced preterm birth.