The life-style habit in question is drug use, and the advice to pregnant women, unsurprisingly, is: don't take anything if you can help it, and make sure your physician knows everything you do take, Familiar? Yet 97 percent of pregnant women were found in the 1970s to use prescription drugs (astoundingly, an average of five-to-ten each), and the use of nonprescription drugs is presumed to be even higher. Fried, who heads an Ottawa team investigating these questions, begins this unusual, highly specific guide by explaining the concepts of probability and risk, so that readers can decide how much weight to give to the studies cited. Then, on the premise that motherhood and responsibility for a baby's health begin at conception, he describes the growth of the embryo and fetus, emphasizing the time at which various structures and systems develop. ""The effect of many substances is critically dependent upon when the unborn baby is exposed to them"": in the case of the thalidomide tragedy, effects on babies' arms, legs, or other structures could be correlated exactly with the time at which the mother took the drug. Finally, Fried deals separately with the effects of alcohol, caffeine, cigarettes, marijuana, and common over-the-counter and prescription drugs. The text can be heavy-going (in babies of alcoholic mothers, ""the first feature is the small head circumference, which is also an indicator of retarded growth. The region of the eyes often displays a number of anomalies. . . ."")--too heavy-going for mothers content with the general advice proferred by standard pregnancy guides. But those interested in the latest research findings, presented in a logical (if technical) framework, will find the effort required worthwhile.