Unsaturated fatty acids with the trans configuration (TFA) are recognized as the most harmful type of fatty acids. The main human source of TFA are foods containing hardened plant oils. Evidence is aplenty that TFA are responsible for increased risk of cardiovascular diseases, tumors, type 2 diabetes, and inflammatory conditions. Due to the harmful action of TFA it is of particular importance to determine the exact effects of TFA on pregnancy and fetal development which will in consequence affect the health of adults. It is believed that the inhibited synthesis of polyunsaturated fatty acids in the presence of TFA is among factors that determine birthweight, duration of pregnancy, and development of the nervous system during intrauterine life and after birth. Pregnant women consuming significant amounts of TFA are at risk of pregnancy-induced hypertension, preeclampsia, and insulin resistance. Reports on the harmful action of TFA on humans persuasively reveal the need to limit their intake, particularly during pregnancy and lactation.