Background. Preterm delivery remains a substantial healthcare problem, complicating 5–10% of pregnancies, and is the major cause of perinatal morbidity and mortality in the developed world. Few effective methods to prevent preterm delivery have been identified to date. Objective. To review systematically the evidence from randomized controlled trials with respect to the hypothesis that increased consumption of marine n-3 fatty acids in pregnancy can prevent preterm birth. Setting. Electronic searches of the following databases were performed: PubMed (1995–2009), SCOPUS including EMBASE (1995–2009), and Cochrane Library. A combination of key words and text words related to fish oil, marine n-3 fatty acids, fish consumption, preterm birth, preterm delivery, prematurity, pregnancy duration, gestational age, parturition, delivery and pregnancy were used. Methods. A systematic review of randomized controlled trials of relevance was conducted. Three trials were included, comprising 921 women for whom data on gestational age and 1 187 women for whom data on birthweight were available. Results. Overall, 46 (8.9%) of 516 women who received n-3 fatty acids gave birth before 37 completed weeks of gestation, compared with 66 (16.3%) of 405 in the control group [relative risk 0.61; 95% confidence interval (CI) 0.40–0.93; p<0.05]. Data on delivery before 34 completed weeks showed the same trend (relative risk 0.32; 95% CI 0.09–0.95). Overall, the mean birthweight was 71g higher in women who received n-3 fatty acids during pregnancy (95% CI 4.73–138.12; p<0.05). The rate of low birthweight was not statistically significantly different between the intervention and the control groups. The mean gestational age at delivery was significantly higher by 4.5days in the intervention group supplemented with n-3 fatty acids compared with placebo (95% CI 2.3–6.8; p<0.05). Conclusions. Marine n-3 fatty acids administered in pregnancy reduce the rate of preterm birth and increase birthweight.