Unveiled in 1960, the birth control pill revolutionized contraception. Yet despite an abundance of birth control options today, almost half the pregnancies in this country are unintended, according to the Centers for Disease Control and Prevention, more than in any other developed nation. What's the reason?

The issue is not technology. But economics and human behavior are another story. Nearly a third of women who start a new type of birth control stop within a year, according to one recent study, largely because of changes in their insurance coverage. All methods have some side effects. And the current crop of intrauterine devices, or IUD's, despite having a nearly perfect efficacy rate, have been slow to catch on, experts say, partly because more doctors need to be trained in inserting them.

"Adherence is a huge issue," said Dr. Nancy Padian, an epidemiologist at the University of California, San Francisco, who specializes in reproductive health and H.I.V. prevention. "To have a significant effect you have to use a product very consistently."

As a result, whether to promote their particular brand or to encourage better compliance, many birth control manufacturers and doctors are promoting the noncontraceptive benefits of contraception. Whether it's reducing the risk of cancer, improving the health of mothers-to-be, easing cramps or enhancing complexion, it's nice to have a medical excuse for using birth control.

Hormonal contraceptive methods use manufactured estrogen and progestin in different combinations and deliver them in a variety of ways—through pills, shots, skin patches, implants, IUDs and vaginal rings. Studies have shown that all those methods reduce the risk of ovarian and endometrial cancer. Some may also help protect against osteoporosis.

Using contraception can also give women a chance to get healthy before they conceive—to stop smoking, lose weight or lower their blood sugar.

If those reasons aren't enough, ads for pills like Yaz cite beautiful skin and relief from premenstrual bloating and mood swings. Loestrin 24 Fe includes an iron supplement. The oral contraceptives Seasonale and Seasonique reduce menstrual cycles to three and four times a year. And Lybrel is supposed to eliminate menstruation altogether, a potential boon to women who suffer from severe cramping and bleeding, anemia, mood swings and migraines that can worsen with menses.

But the pill was such a radical departure that its inventors decided to include a week of placebos in the 28-day pack to create a fake period, or withdrawal bleed. The cycle of 21 days on, 7 days off was pure marketing, designed to make the pill more acceptable to the public. Many women have been controlling their periods for years by skipping the placebos in their packs and taking the real pills continuously.