Women of all ages will soon be able to buy emergency contraceptives over the counter without a prescription, now that the Obama administration has decided to stop fighting a judge's order to make the drugs more easily available.

But better access to emergency contraception doesn't necessarily reduce rates of unintended pregnancy, research has found. Why that's so remains unclear, although researchers have some ideas.

About half of all pregnancies in the United States are not planned, and rates of unintended pregnancy are highest among teenagers. Unintended pregnancies can cause health problems for mother and child, because the woman may not be in the best of health and may not get timely prenatal care.

Last week, the Food and Drug Administration decided that women younger than 17 will no longer be required to have a doctor's prescription to get emergency contraceptives. Women and teenage girls will be able to buy the Plan B One-Step version of the medication without showing an ID.

This change should increase use of the contraceptives, which delay or inhibit ovulation and must generally be taken within three days of unprotected sex. (One prescription-only pill extends that time frame to five days.)

Although making emergency contraceptives easier to get may help individual women buy them, it may not affect the rates of unintended pregnancy rates overall.

Why is that?

"Several studies have shown that even when emergency contraceptives are made more available and financial barriers are removed, there's no impact on unintended pregnancy," says Dr. Eve Espey, chair of the committee on underserved women for the American Congress of Obstetricians and Gynecologists and a professor at the University of New Mexico School of Medicine.

Part of the explanation is that women don't use emergency contraceptives every time they have unprotected sex, says Espey.

Another reason is that estimates of the effectiveness of some types of emergency contraceptive pills may be too optimistic. They prevent up to two-thirds of pregnancies — not all of them.

Although it's not as often discussed, the most effective form of emergency contraception is an intrauterine device. Inserted following unprotected intercourse, an IUD can prevent 95 percent of pregnancies.

IUDs are also effective at preventing pregnancy more routinely. Once inserted, IUDs provide protection for up to 10 years. But IUDs have to be inserted by a doctor or other health care professional.

"Emergency contraception does require some sort of game-time decision," says Bill Albert, chief program officer at the National Campaign to Prevent Teen and Unplanned Pregnancy. "That's the great advantage of long-acting IUDs. You set it and forget it."

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