In the hands of Katherine Oettinger--a former chief of the Children's Bureau, Dept. of Health and Welfare--a statistic is an accusation to be hurled; this might work, except that she has so many statistics at her fingertips, and so many of them are startling, that the end result is numbing. Most revealing: one girl in four has a baby before the age of 20; half are illegitimate, many more born into forced marriages. As recent media exposure has revealed, teenagers are experimenting with sex in droves (again, the percentages are played for all they're worth); but they are not using contraceptives, at least until a skipped period scares them. Reasons vary; chief among them are ignorance, fear of disclosure to parents, worry that contraceptives are ""wrong,"" embarrassment in discussing the matter with one's partner, and even the ""baby-doll"" syndrome--the prestige and chance to lavish affection that a baby represents to a teenaged girl. Oettinger has the dimensions of the problem down pat, but she's weak on solutions: better communication, a less judgmental attitude on the part of parents, better sex education, suggested readings--inarguable staples all. In sum, an eye-opening topic--and an eye-catching title--but a shrill, arid follow-through.