Dietary long-chain n-3 fatty acids (FA) may prolong gestation by inhibiting formation of prostaglandins from arachidonic acid. FA were quantified in phosphatidylcholine (PC), phosphatidylethanolamine (PE), and total lipids (TL) of red cells sampled during pregnancy from 29 Faroese women. The ratio of long-chain n-3 FA to arachidonic acid (the (3/6) ratio) was used as the most relevant single measure of exposure. In 18 women with certain gestational age and with spontaneous onset of delivery, gestational age was significantly associated with the (3/6) ratio quantified in PC (correlation coefficient 0.50, P = 0.035), but not with the (3/6) radio in PE (correlation coefficient 0.21, P = 0.40) or TL (correlation coefficient 0.29, P = 0.26). The association with the (3/6) ratio in PC could not be attributed to confounding by maternal age, weight, marital status or smoking.