All of the essential n-6 and n-3 fatty acids accumulated by the fetus must be derived by transfer from the maternal circulation, and ultimately must originate from the maternal diet. After birth, the breast-fed infant receives essential fatty acids via mother's milk, or human milk substitutes and later complementary foods. Trans fatty acids (TFA) may have adverse effects on growth and development through interfering with essential fatty acid metabolism, direct effects on membrane structures or metabolism, or secondary to reducing the intakes of the cis essential fatty acids in either mother or child. TFA are transported across the placenta and secreted in human milk in amounts that depend on the maternal dietary intake. Inverse associations have been shown between TFA and the essential n-6 and n-3 fatty acids in newborn infants, human milk and preschool children. This support the need to reduce industrially produced trans fatty acids (IP-TFA) and improve dietary fat quality, particularly by increasing intake of n-3 fatty acids.