WEDNESDAY, May 9 (HealthDay News) -- Many women who need emergency contraception after unprotected sex are aware of the "morning-after" pill as an effective way to prevent unwanted pregnancy.

Fewer may be aware that the intrauterine device (IUD) can also work as an emergency contraceptive. In a new review, researchers find that it is safe and actually more effective for emergency contraception than the morning-after pill.

Although IUDs have been studied for at least 35 years for emergency contraception, the use is not well known, said researcher James Trussell, a professor of economics and public affairs at Princeton University. "Our guess is that most gynecologists do not mention IUDs for emergency contraception," he said.

Trussell's review, which evaluated 42 studies on IUDs, is published online May 9 in the journal Human Reproduction. The studies were conducted in six countries from 1979 through 2011. More than 7,000 women were using eight different kinds of IUDs.

The IUDs, when used for emergency contraception, had a failure rate of less than one per thousand, or 0.1 percent, Trussell said. In comparison, other studies have found a failure rate of some morning-after pills is 2 or 3 percent. And some research has found that the pills don't work in women who are overweight, with a body- mass index of 26 or higher.

The new analysis reaffirms what is known about IUDs, said Dr. Jill Maura Rabin, chief of the division of ambulatory care and head of urogynecology at the Long Island Jewish Medical Center in New Hyde Park, N.Y.

However, she said, the IUD is not the best emergency contraception choice for every woman. It works best, she said, for a woman who decides after unprotected intercourse that she wants long-term but reversible birth control. IUD insertion requires a doctor's appointment, and is much more expensive than the morning-after pill, she said.

Some, but not all morning-after pills are available without prescription to women aged 17 and older.

According to Planned Parenthood, morning-after pills cost from about $10 to $70. IUD insertion, which requires a doctor's office visit, can cost $500 or $1,000, according to Planned Parenthood.

For any form of emergency contraception, the earlier it's done the better. In the studies reviewed by Trussell, the time from unprotected sex to IUD insertion ranged from two days to more than 10 days, but he said within five days is best.

An IUD is a T-shaped piece of plastic, inserted into the uterus via the cervix. One sold in the United States, ParaGard, releases copper. The other, Mirena, releases the hormone levonorgestrel. The levonorgestrel system has not been studied for use as an emergency contraceptive, as the copper model has, according to Trussell.

Exactly how the IUD works isn't certain. Experts say it may prevent the sperm and egg from joining. Or, it may change the lining of the uterus so that a fertilized egg can't attach.

Recently, the use of IUDs in the United States has increased, the researchers said, with about 5 percent of women at risk of pregnancy using it. In the 1970s, the device fell out of favor after safety fears linked with an IUD known as the Dalkon shield, which was withdrawn from the market.

However, IUDs on the market today are improved, the authors said. The American Congress of Obstetricians and Gynecologists and other organizations consider them safe for most women.

The morning-after pill contains hormones that may work by thickening cervical mucus so the sperm and egg can't join or by affecting the lining of the uterus as the IUD does. The morning-after pill is not an abortion pill. Makers of the pills (such as Plan B, ella) provide timing instructions for users.

Women should know that certain problems will prevent them from being able to get an IUD inserted soon after unprotected sex, Rabin said. For instance, a woman who is found to be already pregnant could not get an IUD, of course. An active pelvic infection, such as chlamydia or gonorrhea, would also prevent safe insertion of an IUD, she said.

More information

To learn more about emergency contraception, visit Planned Parenthood.