“Emergency contraception” has almost become synonymous with “morning after pill.” But another type of emergency contraception — the placement of an intrauterine device — is even more effective.

A new review of 42 studies of IUDs for emergency contraception found that IUDs inserted within five days of unprotected intercourse prevented pregnancy more than 999 times out of 1,000 — a failure rate below 0.1 percent.

Morning-after pills, by contrast, have failure rates 10 to 30 times higher (1 percent to 3 percent), depending on the type of pill chosen.

But IUDs are rarely offered as an emergency contraceptive, especially in the United States, write the authors of the new report in the journal Human Reproduction. They cite a recent study in California that found 85 percent of contraceptive providers there never recommended IUDs for emergency contraception.

There are lots of reasons pills are more popular than IUDs, which are typically small coils coated with copper and which prevent fertilized eggs from implanting in the uterus.

One barrier: Many providers require two visits for an IUD insertion. Price is a second barrier. Morning-after pills typically cost $10 to $70, whereas an IUD costs $500 and up, according to Planned Parenthood.

Also, an IUD will continue to prevent pregnancy for a decade or longer. Many women of child-bearing age aren’t interested in such a semi-permanent solution.

Another factor cited by the researchers: IUDs still suffer a safety-image problem. In the 1970s, one IUD, the Dalkon Shield, was poorly designed, leading to bacterial infections and sepsis in many users. Some 300,000 lawsuits were filed against the company making the device.

But today’s IUDs are much safer, write the report’s authors, who hail from Princeton University and universities in England and China. In the United States, rates of IUD use among women of child-bearing age have pushed up from under 1 percent in 1995 to nearly 5 percent in 2010.

The Chinese, in particular, have embraced IUDs. The article’s authors found that 43 percent of women in China who use contraception use IUDs.

The report’s authors urge providers to consider recommending IUDs for emergency contraception, especially for women not interested in becoming pregnant for several years. After all, according to the Guttmacher Institute, half of all pregnancies in the United States are unintended.