Prostaglandins, especially prostaglandin F2 alpha, are important regulators in the onset and maintenance of parturition in humans. Inhibition of prostaglandin formation by drugs can prolong gestation in humans. High dietary intake of long chain (n-3)-fatty acids can inhibit formation of many arachidonic acid-derived prostaglandins as well as stimulate formation of eicosapentaenoic acid-derived prostaglandins. The latter ones often have lower biological activity than the former ones. The effect of (n-3)-fatty acids on prostaglandin formation has been shown by analysis of prostaglandin metabolites in human urine. Very little is known about the tissue origin as well as the physiologicalfunctions of those prostaglandins, the metabolites of which are quantitated in urine from non-pregnant humans. In pregnant women the increased amount of prostaglandin F2 alpha-metabolites in urine, plasma and amniotic fluid probably is of uterine origin. Women in the Faroe Islands have longer gestation and are giving birth to infants with higher birth weight than women in Denmark. The Faroe Islands is a fishing community with an old cultural tradition for eating pilot whales. Fish and marine mammals are rich in long chain (n-3)-fatty acids. We have hypothesized that a high intake of long chain (n-3)-fatty acids prolongs gestation in humans by interfering with uterine prostaglandin formation, possibly by inhibiting formation of prostaglandin F2 alpha and prostaglandin E2. Preliminary results of an epidemiological observational study have shown a positive correlation between (n-3)-fatty acids in maternal erythrocytes and length of gestation. These results are in accordance with our hypothesis, but a causal relationship remains to be shown.