It was hypothesised from previous observational studies in the Faroes and Denmark that dietary marine n-3 fatty acids would prolong the duration of pregnancy and thereby increase the birth weight, by influencing the production of prostaglandins involved in the process of parturition. Five hundred and thirty-three healthy Danish women were randomly assigned in the 30th week of pregnancy to fish oil (2.7 g n-3 fatty acids (4 1-g capsules Pikasol oil) per day), olive oil (4 1-g capsules per day) or no oil supplementation. The three groups differed in mean gestational age at birth (ANOVA, p = 0.006), with the fish oil group ranking highest and the olive oil group lowest. Babies born to women allocated fish oil had on average 4.0 (95% confidence interval 1.5;6.4) days higher gestational age and 107 (95% confidence interval 1;214) g higher weight at birth than babies born to women allocated olive oil; the difference in gestational age depended on the level of fish intake at enrollment, with a low fish intake enhancing the difference. Fish oil supplementation in the third trimester seems to prolong gestation while allowing continued growth of the foetus; this effect seems to explain the difference between the Faroes and Denmark in pregnancy duration.