Seven in 10 never-married U.S. women aged 15-19 have had a sex education course, almost all of them in school. Young black women are slightly more likely than whites to have had such a course. About half of all never-married teenagers have had courses which included some teaching about modern contraceptive methods; six in 10 have had some formal instruction about VD, and seven in 10 have had courses that included detailed instruction about the monthly menstrual cycle. Six in 10 of those who have had a course with information on the menstrual cycle claim to know the period of greatest pregnancy risk; but only one-third of those who have had such a course can in fact correctly identify the time when the risk of pregnancy is highest. Young white women are nearly two times more likely than blacks to identify the period of risk correctly. Knowledge of the period of risk is a particularly important issue in relation to teenage pregnancy, because many sexually active teenagers say that they do not use contraception because they believe they are protected from the risk of pregnancy by the time of month when they have intercourse. Among those who claim to know the period of greatest risk, a sex education course is the most common source of information for both blacks and whites, with home a distant second. Medical personnel and facilities convey information about pregnancy risk the most effectively, but they are the first source for only a tiny fraction of teenagers.(ABSTRACT TRUNCATED AT 250 WORDS)