Because of the uncertainty, the Surgeon General of the United States and the National Council on Alcoholism now recommend that pregnant women remain abstinent. The immense variety of symptoms proves that the effect of alcohol on the fetus is pervasive, but its mechanisms are difficult to identify. Alcohol might interfere with the metabolism of zinc and magnesium or reduce the capacity of the placenta to carry food and oxygen to the fetus. It might alter the level of various hormones, including corticosteriods, and growth hormone. But the most important effect is a change in the molecular structure of cell membranes that alters their size and shape, retards their growth and division, and generally interferes with the organization of tissue.
Any organ is most sensitive when it is developing most rapidly.
Alcohol affects the brain most powerfully in the early stages of growth, when nerve cells are migrating toward the surface of the body to form the cerebral cortex. Scrambled layers of brain cells known as heterotopias are found in FAS children and in the offspring of animals exposed to high doses of alcohol early in pregnancy. In one study, monkeys were given alcohol from either the first or the fifth week of pregnancy until delivery. The ones who were exposed to alcohol early in pregnancy had more severe and more uniform symptoms, even though the others were given higher doses. Alcohol can distort all these processes and alter the general shape and size of the brain.