"Pregnancy obviously does have an overwhelming impact on the woman. The most readily observable impact of pregnancy, of course, is that of carrying the pregnancy for nine months. Additionally there are numerous more subtle but no less drastic impacts. Without the right to respond to unwanted pregnancy, a woman is at the mercy of possible contraceptive failure, particularly if she is unable or unwilling to utilize the most effective measures. Failure to use contraceptives effectively, if pregnancy ensues, exacts an exceedingly high price."