And then there is the fact that many women may not know what an IUD is, or that such an option exists. IUDs are not nearly as highly advertised as birth control pills, doctors said -- at least not yet.
"The reason most women choose pills is a combination of marketing, patient familiarity and comfort level, physician comfort with counseling and insertion, and misconceptions about IUD safety," Streicher said.
But as more studies like this most recent one emerge, Streicher said, more women may shift to IUDs in the years to come.
"Very clearly, contraception that is not user-dependent is going to have the lowest failure rates," Streicher said. "Half of unintended pregnancies every year are not 'no contraception,' they are 'failed contraception' such as missed pills, etc."
Vonder Haar, a participant in the study, has now had an IUD for three years, and she said she is grateful to the study for giving her the opportunity to use this method of birth control.
"I have recommended this to everyone," she said. "It has made such a difference; I think every girl should be able to have access to this."