Throughout pregnancy, Lewis rats were fed standard rat chow supplemented with 15% (w/w) of either MaxEPA fish oil (FO) or arachis oil (AO); a third group was fed standard rat chow only (St) (n = 15, 15, and 16 rats, respectively). Compared to AO-rats, FO-rats had substantially higher levels of n-3 fatty acids and lower levels of n-6 fatty acids in maternal and fetal tissues at delivery. FO-rats had a longer average gestational age than AO-rats (p <0.01), primarily due to a high proportion of AO-rats with short gestations. Birthweights of FO-rats were lower than those of AO-rats (P <0.05), which may be related to the lower maternal weight gain during pregnancy and the lower food consumption observed in FO-rats (p <0.01). Litter size and occurrence of stillborn pups were not significantly different in the groups (p > 0.1). The findings are compatible with the hypothesis that gestational age is positively related to a high dietary intake of n-3 fatty acids.