The fatty acid composition in cervical mucus was determined in 37 pregnant and 17 nonpregnant women using gas liquid chromatography. In both groups palmitic acid (16:0) and oleic acid (18:1) were the predominant acids comprising more than half of the total amount. Compared to nonpregnant controls, pregnant women had markedly elevated levels of oleic acid (18:1), while mean levels of myristic acid (14:0) and stearic acid (18:0) were significantly lower. These pregnancy-induced changes in fatty acid pattern could not be ascribed to the increased viscosity of cervical mucus. Only minor differences in fatty acid pattern were found between women in first trimester and at term. In contrast to nonpregnant controls, a significant negative correlation between levels of myristic acid (14:0) and docosahexaenoic acid (22:6) was found in pregnant women, indicating a redistribution of these two fatty acids during pregnancy. The present study demonstrates that pregnancy alters the fatty acid composition of cervical mucus lecithin, and that the characteristic changes are present before the 10th gestational week.