Intrauterine devices (IUDs) are amazingly, fantastically good at preventing pregnancy — better than pretty much any other available contraceptive.

Birth control pills, which have to be taken regularly, are susceptible to human error. The pill has a 6 percent failure rate. So out of 1,000 women taking birth control pills, 60 will become pregnant in a typical year. Among women who use an IUD, that number will be between 2 and 8 (depending on the type of IUD they use).

The American College of Obstetricians and Gynecologists recommends IUDs and the contraceptive implant (the one other long-acting, reversible contraceptive) as a "first-line" contraceptive that should be "encouraged as an option for most women."

It should be no surprise, then, that IUD use has hugely increased in recent years. New federal data show that the use of long-acting contraceptives, like IUDs and the birth control implant, has quadrupled since 2002.

An estimated 11.6 percent of American women now use long-acting, reversible contraceptives like IUDs. Still, the devices tend to get an especially bad rap in the United States because of the Dalkon Shield, an early IUD from the 1970s. It was hard to insert (it looked like this), sometimes failed to prevent pregnancy, injured as many as 200,000 women, and sometimes led to infertility or even death. All in all, it was a terrible contraceptive that was subsequently pulled from the market.

Today's IUDs are different: They're safer, easier to insert, and incredibly effective. That probably explains why 40 percent of gynecologists using a contraceptive are using IUDs — way more than the general population. Here are a few of the most common questions about how they work, answered. If there are some we missed, feel free to email me here.

1) How do IUDs work?

IUDs make the uterus 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 they 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 Mount Sinai Hospital in New York. 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. "Levonorgestrel 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 effective for 12 years, while Mirena and Skyla last five and three years, respectively.

2) How is an IUD inserted?

It's done in four steps that take about five minutes.

Step one is pretty much like a typical pelvic exam that happens at a regular checkup. With the patient's feet in stirrups, the doctor inserts a speculum into the vagina.

Step two is where the health care provider straightens out the cervical canal to create an easy path for inserting the IUD. This is done using a clamp called a tenaculum to hold on to the cervix. MacIsaac said this usually feels like a "pinch" and can be similar in pain to a menstrual cramp.

Step three involves measuring the size and angle of the uterus, to make sure that the IUD stays put after it's inserted. To do this, the provider, with the tenaculum still holding the cervix straight, inserts a straw-size rod into the uterus. This is called "sounding" the uterus, and providers do it to "know how far you have to go in order to insert the device," said Vanessa Cullins, vice president for medical affairs at Planned Parenthood.

The final step is inserting the IUD itself. The measuring rod comes out of the cervix, and another rod that contains the device goes in. The health care provider uses this rod to position the IUD in the cervix and then leave it there.

That, in four steps, is how IUDs find their uterine homes. You can watch an animated video of the process here, if that's the kind of thing you're into.

3) How do I choose the right IUD?

Probably in consultation with your health care provider. But here's a quick chart comparing the differences between the three IUDs available in the United States.

The big difference between IUDs is mostly their mechanism of action — whether they use copper or hormones to prevent pregnancy — and how long they last. Both have low failure rates, although hormonal IUDs have the lower failure rate of the two.

4) Does getting an IUD hurt?

The frustrating answer here is that it depends. Some women say it hurts a lot; others barely feel a thing.

"I've seen such a huge range," said MacIsaac. "I've done both ones where people are like, 'I don't feel anything,' and others where people say they feel like they're going to throw up because it hurts so much."

There is one study of Swedish women who had never before given birth and had an IUD inserted. It found 9 percent experienced "no pain," 72 percent said it was "moderately painful" and 17 percent said it was "severely painful." And, for what it's worth, four months after the insertion, 5 percent said they were dissatisfied with their new method of birth control.

In one study, 72 percent of women said getting an IUD was "moderately painful"

In a separate study, about two-thirds of women rated the pain similar to the cramps they experienced during a menstrual cycle. Other surveys have found that women tend to anticipate IUD insertion to be more painful than the actual experience.

A lot of providers will recommend taking an over-the-counter anti-inflammatory like Motrin or Advil an hour before the procedure. For patients who experience higher pain levels, MacIsaac sometimes administers a low dose of Valium.

After insertion, women can expect to have cramps for about 24 hours or so. These are usually similar in pain level to menstrual cramps and subside after about a day.

5) Is there any chance I can get pregnant while I have an IUD?

It's tiny, but, yes, there is chance. Of 1,000 women using Paragard (the copper IUD) typically eight will become pregnant in a given year. For 1,000 women using a hormonal IUD (either Mirena or Skyla), two will have IUDs that fail.

This can happen for two reasons. There's the possibility that a woman could have an expulsion and not realize it. This is especially true for women using Paragard, who often experience heavy bleeding after insertion and heavier periods during the first few months or year.

"The first two periods after Paragard you might have a blood clot the size of a half-dollar," said MacIsaac. "So the IUD could fall out in the toilet, and you may not notice." (We'll get to a way to prevent this in just a moment.)

"There's, like, a billion sperm every time a man ejaculates, and they're really stubborn"

And even if the IUD stays in, there's a small chance that somehow, some way, a very persistent sperm will manage to still swim its way through the IUD's best defenses. Or as MacIsaac put it, "There’s, like, a billion sperm every time a man ejaculates, and they’re really stubborn."

With the copper IUD, for example, there's that teeny, tiny chance that the copper ions that block sperm from entering the uterus left a tiny bit uncovered. And a sperm happened to make its way to the egg.

To repeat, this is rare. But it does happen, even when a doctor has inserted an IUD perfectly.

6) What are the possible risks of an IUD?

Expulsions and perforations are the two main complications. Both are rare and perforations are a much more serious problem.

Expulsions happen when the body pushes the IUD out of the uterus. This can happen if health care provider doesn't insert the device correctly — or, even if it is placed correctly, the uterus becomes irritated with the foreign object and rejects it.

The American College of Obstetricians and Gynecologists estimates that IUDs have an expulsion rate between 3 and 5 percent.

Expulsions aren't necessarily dangerous to the body, although women may feel some discomfort and cramping if the IUD starts to come out of the uterus and through the cervix.

"Whenever the uterus is trying to expel something, whether that's a baby, or menstrual blood, or an IUD, it cramps," said Cullins.

The biggest risk factor of an expulsion is pregnancy: Women become fertile within about a day of their IUD coming out.

Perforations occur when the IUD punctures the uterine wall. This can be incredibly dangerous, possibly damaging the nearby bladder or intestine. If left untreated, perforations can lead to hemorrhaging or sepsis.

Perforations are extremely rare, much rarer than expulsions. The largest study on uterine perforations, conducted in 2007 on more than 60,000 European women, found a perforation rate of 0.1 percent for both hormonal and copper IUDs.

There are no known long-term health risks of IUD use. The IUD does not have any effect on long-term fertility; women can typically become pregnant within one day of having the device removed. And some research has actually shown IUDs could have a protective effect, as users have lower rates of gynecological cancers.

7) Who should get an IUD?

The American College of Obstetricians and Gynecologists recommends IUDs for the vast majority of women who are sexually active, of reproductive age, and who do not want to become pregnant. They are the "most effective forms of reversible contraception available and are safe for use by almost all reproductive-age women."

Gynecologists recommend IUDS for all reproductive-age women

ACOG's recommendation mostly comes down to efficacy: IUDS are just way better at preventing pregnancy than nearly any other contraceptive. (The only thing with a lower error rate is the birth control implant, which you can read more about here.) There's no space to screw up the IUD like there is with birth control rings, patches, and pills that have to be taken at regular intervals.

As ACOG points out, "More than half of women who have an unplanned pregnancy were using contraception. ... [IUDs] have the highest continuation rates of all reversible contraceptives, a key factor in contraceptive successes."

The one group of people who should not get IUDs is those who have an untreated sexually transmitted infection (STI). Infections can lead to a higher risk of pelvic infection when the device is inserted. This is why many providers will do an STI test at the time of insertion.

8) Do I need to do anything after the IUD is in?

Not much — but there are a few things MacIsaac recommends to patients. For starters, no unprotected sex for two weeks after insertion. That's because right after the IUD is inserted, there's a higher risk that any sexually contracted disease could travel up into the uterus and become a pelvic infection. But after about two weeks, that risk goes away.

MacIsaac also has patients check whether they can feel the string from their IUD for first six months or so after insertion. This is the string that she will eventually use to remove the IUD, and it typically can be felt by the patient where it hangs just below the cervix. The reason she has patients check for the string is to make sure the IUD hasn't fallen out accidentally, which doesn't usually happen — but if it does, it tends to be shortly after insertion.

And if they can't feel the string, it's not an immediate cause for panic. "Many women aren't able to reach their cervix, or the strings are tucked up behind it," MacIsaac said. "If after a few months they can't feel them, they can come in and we'll do an ultrasound to make sure it's still there."

9) Can I or my partner feel an IUD during sex?

Not if it's situated correctly. But if the IUD is falling out during an expulsion, it's possible that a woman or her partner could feel it during intercourse or other sexual activities — and that would be reason to head back to the doctor for a visit.