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Another reason to love IUDs: they last even longer than we thought

Intrauterine devices, or IUDs, are one of the most effective types of birth control we have right now. They prevent pregnancy among 99.2 percent of their users, a way better success rate than condoms or birth control pills.

Now, researchers have found even more good news about IUDs: they last longer than government regulators think. A new study in the journal Obsetrics and Gynecology finds that Mirena, the IUD that uses hormones to prevent pregnancy, works for at least six years — even though the Food and Drug Administration guidelines say it only lasts five.

This same study also found that birth control implants — hormonal contraceptives that a doctor inserts into a patient's arm — also last one year longer than the government has approved for use. The Food and Drug Administration says the implant lasts for three years; researchers found that women who used it for a fourth year did not have any pregnancies.

Time's Alexandra Sifferin summarized how the study worked:

All the women were between the ages of 18 and 45 and their contraceptives were within six months of expiring before they enrolled in the study. The women were informed of the pregnancy risk associated with using their device longer than recommended, and the researchers called them for followup every 6 months for 36 months or until the women had their device removed.

By the end of the trial period, none of the women using the implant were pregnant and there was one pregnancy among the women using the IUD. Still, the failure rate was similar to the failure rate of the IUD when used during the five-year period (which is under 1%).

This isn't the first study to suggest that long-acting, reversible contraceptives could last even longer than the federal government says. A literature review published in 2014 found "good evidence" that the Mirena IUD can work for as long as nine years. Most of their data came from women over 25 who had previously had children.

The copper IUD, ParaGard, is already approved for longer use: the Food and Drug Administration says it lasts 10 years.

Longer-lasting contraceptives could have important public-health consequences. While the Affordable Care Act (Obamacare) eliminated co-payments for all contraceptives, the devices are still expensive for uninsured women (Planned Parenthood has found that some providers charge as much as $1,000 for the device). If IUDs lasted longer and needed fewer replacements, that could reduce costs and contraceptive use overall — making the devices even better at reducing unplanned pregnancy.

As for what doctors and patients should do with this information, the authors of the 2014 article did have recommendations for IUD users over 25 who had previously had children (the group where researchers have the best data). In that case, they write, "clinicians should counsel that extended IUD use is currently an off-label practice, but likely highly time if IUD insertion.

How the IUD works

IUDs are small devices inserted into the uterus that make it an incredibly hostile, inhospitable, sad place to be a sperm — which prevents a sperm from getting anywhere close to an egg.

This is true for the two types of IUDs, copper and hormonal — although the way the two of them work is a little bit different.

Paragard, the copper IUD, "releases copper ions in a steady, slow fashion so that the uterine cavity is bathed in those copper ions," said Laura MacIsaac, director of the family planning program at Mt. Sinai Hospital in New York City. She also helps the American College of Obstetricians and Gynecologists develop policy on birth control methods.

The copper ions, she explained, "kill the sperm. It's almost like a barrier method, like a condom, that keeps the sperm from getting any further than the uterus."

I asked MacIsaac if this was something akin to the IUD creating a force field around the uterus, and she said that was a pretty fair analogy. So if that helps you think about how an IUD works (or if, like me, the concept of a uterine force field amuses you), you're welcome.

Mirena and Skyla, the two hormonal IUDs, use a hormone called levonorgestrel to do something similar. "Levonogesterel thickens the cervical mucus, which is the first step to sperm getting through the cervix [and then to the uterus]," MacIsaac said. "It also makes the whole lining of the uterine cavity very thin and unfriendly for sperm transport."

One other key difference between all the three IUDs available in the United States is how long they work. The copper IUD is approved for use for 10 years, while Mirena and Skyla are approved for five and three years, respectively.

You can read more about IUDs — how they're inserted, possible side effects, and why gynecologists love them so much — here.