Researchers found that of more than 4,100 women who were seeking birth control, about 45 percent overestimated the effectiveness of the Pill and condoms.

They also had too much faith in hormonal birth control patches, vaginal rings and injections, according to findings reported in the American Journal of Obstetrics and Gynecology.

The findings point to a need for better education on how well different birth control methods work with "typical use" in the real world, study leader Dr. David L. Eisenberg told Reuters Health.

In the U.S., the Pill and condoms are the most popular reversible forms of birth control. But they are not the most effective.

That designation goes to intrauterine devices (IUDs) and contraceptive implants.

IUDs are implanted in the uterus, where they release small amounts of either copper or the hormone progestin to prevent pregnancy. The contraceptive implant, about the size of a matchstick, is inserted under the skin of the arm, where it releases controlled amounts of progestin.

The hormonal IUD, sold under the brand-name Mirena, can prevent pregnancy for five years, while the copper version, sold as ParaGard, is effective for about 10 years. The contraceptive implant (Implanon) works for three years.

It's estimated that between 0.2 percent and 0.8 percent of women who use an IUD will have an unplanned pregnancy within a year. The rate is just 0.05 percent with a contraceptive implant.

The advantage is that unlike birth control pills and condoms, the IUD does not rely on perfect use.

With the Pill, the pregnancy rate with "typical use" is about nine percent per year. With condoms, it's between 18 and 21 percent.

"We need to do a better job of educating the public -- women and men -- on the failure rates with typical use," said Eisenberg, of Washington University in St. Louis School of Medicine.

People also need to know, he said, that IUDs and the contraceptive implant are the most effective type of reversible birth control. (Surgical sterilization is also close to 100 percent effective, but it's permanent.)

But it's not only the public that needs more knowledge on contraception, Eisenberg said. Doctors, too, may not be fully informed, or may not be comfortable with inserting an IUD or implant.

A recent survey by the U.S. Centers for Disease Control and Prevention (CDC) found that 30 percent of health providers doubted the safety of IUDs for women who'd never given birth.

When IUDs first came out, there were concerns that they might raise the risk of pelvic infection and jeopardize women's future fertility. So the original IUD labeling said the devices were contraindicated for women who'd never had children.

But it's now known that IUDs do not carry those risks.

Still, Eisenberg said, "there are a lot of myths and misconceptions about which women are candidates."

The American College of Obstetricians and Gynecologists has said that IUDs and implants should be offered as "first-line" options for most women, because of their effectiveness and safety.

Yet only about five to six percent of U.S. women who use contraception choose those methods.

Based on the current findings, Eisenberg said, many more women might choose them if they had full information and full coverage of the cost.

The study included 4,144 St. Louis-area women who were surveyed before getting contraceptive counseling. They were asked to rate the effectiveness of different birth control options, picking from a list of choices.

Overall, 45 percent overestimated the Pill, condoms, the hormonal patch, the hormonal vaginal ring and the injection hormone Depo-Provera. (It was not possible to overestimate the effectiveness of IUDs or implants because of the survey's structure.)

Then, after the women were counseled on all their options, a full 71 percent chose an IUD or implant.

That suggests that when you "remove the obstacles," Eisenberg said, many women want those contraceptives.

Besides lack of awareness, those obstacles include cost.

IUDs and implants have a big "upfront" cost: the Mirena IUD itself has gone up over the years and is now nearly $800. The ParaGard price tag is about $500. Then there are the doctor's charges.

The Implanon implant costs between $400 and $800, with all charges considered.

Since the devices last for years, that cost may be worthwhile over time, Eisenberg pointed out. Birth control pills, for instance, cost anywhere from about $10 to $50 a month, depending on whether a woman uses generic or brand-name pills.

But if women don't have full insurance coverage of the upfront cost, they may balk at the price tag.

The devices can have unwanted side effects, too. With the implant, irregular menstrual bleeding is most common; some women stop having their periods altogether.

The Mirena IUD may also cause menstrual irregularities. But it generally makes periods lighter, which is why it is also approved as a treatment for heavy bleeding. The ParaGard IUD has the opposite effect: menstrual bleeding and cramping can increase, though that may go away over time.

Eisenberg, who has already presented some of the current findings at a medical conference, said he thinks IUDs and implants should be the "default" options offered to women seeking reversible birth control.

But that's not what many doctors are doing, he noted.

If women can't get the contraceptives from their own doctor, Eisenberg said, they may be able to find a family planning center that can provide them.

SOURCE: bit.ly/IefOpw American Journal of Obstetrics and Gynecology, online April 9, 2012.

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