One of the most dangerous drugs for pregnant women to consume is alcohol. Because alcohol is delivered quickly into the blood and passes quickly into the tissues and membranes, the human fetus (胎儿) is particularly vulnerable to its effects. In fact, the negative effects on a fetus are so evident that babies born after exposure to alcohol are said to be suffering from fetal alcohol syndrome. As a pregnant woman drinks alcohol, the alcohol is passed into her bloodstream almost simultaneously. Moreover, because the bloodstream of the fetus is inseparably tied to that of the mother, the alcohol passes directly into the bloodstream of the fetus as well. And, what is more, the concentration of alcohol in the fetus is exactly the same as in the mother. For the mother, this concentration is not a problem because her liver can remove one ounce of alcohol from her system per hour. However, the fetus's liver is not completely developed (how developed it is depends on its stage of development). The rate at which it is able to eliminate the alcohol from the blood of the fetus is much slower. Eventually, the alcohol will be returned to the mother's system by passing across the placenta (胎盘), but this process is slow. By the time this takes place, major neurological damage may have already occurred. Research has shown that as little as one drink of alcohol can produce significant, irreversible damage to the fetus. Babies born after exposure to alcohol generally exhibit facial distortion, inability to concentrate, and difficulty in remembering. Simply speaking, it is imperative that pregnant women avoid alcohol. What is the main topic of this reading?