More than 50% of all women in the United States have had sexual intercourse by 18 years of age. Up to one half of these adolescents do not use contraception at first intercourse. Although chronically ill adolescents may differ slightly from these norms, such differences are probably minor. Prevention of adolescent pregnancy remains a challenge in the medical community, and this can be especially difficult for the chronically ill adolescent. In some cases, medications taken for certain chronic conditions may alter the effectiveness of hormonal contraception, and pregnancy in these cases may pose substantial risks to the mother as well as her fetus.