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, can prevent pregnancy for five years, while the copper version, 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.)