We summarize data on the potential interaction of trans isomeric fatty acids [trans fatty acids (TFAs)] with the availability of long-chain polyunsaturated fatty acids (LC-PUFAs) in the perinatal period. Today, TFA intakes in pregnant and lactating women can be estimated to be ∼1% of energy in the majority of the population. The significant inverse associations seen between TFAs and LC-PUFAs in pregnant women in 3 different European populations investigated in a recent study raise doubts about the nutritional adequacy of high TFA intakes during pregnancy. In a recent study on the TFA content of human milk in a sizable group of mothers at the sixth week of lactation, both arachidonic and docosahexaenoic acids correlated significantly inversely to 18-carbon TFAs but not to 16-carbon TFAs, and at the sixth month of lactation arachidonic acid correlated significantly inversely to 18-carbon TFAs but not to 16-carbon TFAs. Similarly, significant inverse correlations were seen between 18-carbon TFAs and arachidonic and docosahexaenoic acids in both artery and vein wall lipids in a sizable group of healthy term infants. The TFA data obtained in umbilical blood vessel wall lipids were related to the neurologic condition of healthy children at 18 mo of age: children with minimal neurologic dysfunction at age 18 mo had significantly higher cord blood vein wall trans octadecadienoic acid values than did neurologically normal children. Total TFA values as well as total 18-carbon TFA values in umbilical vein wall lipids were significantly inversely associated with neurologic optimality score. Contradictory data renders it impossible to draw firm conclusions on the role of TFAs in modifying fetal growth; however, TFA exposure may be a confounding parameter in studies that investigate the relation between fetal fatty acid supply and intrauterine growth.