By Stephanie Stahl

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PHILADELPHIA (CBS) – There is a new birth control warning for women. The pill is not the most effective way to prevent pregnancy, despite millions taking it, according to a new study. 3 On Your Side Health Reporter Stephanie Stahl explains why, and what is more reliable.

Birth control pills are the most popular contraceptive in the United States, but their effectiveness depends on women remembering to take a daily pill and having easy access to refills. And because many women don’t, there are a higher number of unintended pregnancies with the pill when compared to other birth control options.

Twenty-six-year-old Kimberly Casten switched from birth control pills to an IUD because of health concerns.

“I care about the levels of hormones that are circulating in my body, because I know it affects my brain,” said Kimberly.

Switching from the pill to an IUD also means she’s better protected, according to a new study in the New England Journal of Medicine. The study finds that IUDs, or Intrauterine Devices, and hormonal implants are 20 times more effective at preventing pregnancies than short term contraceptives like birth control pills, the patch or the ring.

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“There’s a one time placement and doesn’t require a daily weekly or monthly administration,” said Dr. Adam Jacobs, an ob-gyn.

Hormonal implants, which go in the upper arm, work for three years. There are two kinds of IUDs. Both are inserted into the uterus and can last up to ten years. Despite previous problems, doctors say the IUDs currently available are very safe.

“Women clearly are not at increased risk of infection and are also not at risk of having problems, getting pregnant in the future after getting the IUD removed,” said Dr. Jacobs.

Implants and IUDs can be pricey. Up front costs can be more than $500. Because of that, many women just can’t afford them.

The study showed that when IUDs were offered for free, 75 percent of women chose them. Currently, only five and a half percent of women using contraception use IUDs.

For more on the study, click here.

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Stephanie Stahl