Back in high-school sex ed class, most men learned that pulling out is a no-go if you don’t want impregnate your partner or contract STDs.

So imagine our surprise when we came across a study in the journalConception that claims 33 percent of women have used this this as a form of birth control within the last month!

Now, if used perfectly, withdrawal—pulling out during sex before you ejaculate—can be effective. Couples who use it perfectly have a 4 percent chance of becoming pregnant within a year, research from Princeton University estimates.

But “perfect” is the key word, and using it that way is extremely difficult. With “typical” use, the risk of pregnancy grows to 22 percent.

Perfect use means pulling out early enough so that no semen gets in the woman’s vagina or on her vulva, says Raegan McDonald-Mosley, M.D., M.P.H., chief medical officer at the Planned Parenthood Federation of America.

Typical use—which is much more common—refers to pulling out too late, due to a lack of awareness or control of your ejaculation. And this, of course, significantly raises the risk of pregnancy.

Your success with pulling out depends on your ability, motivation, and willpower to do it perfectly every single time, says Dr. McDonald-Mosley.

That’s tough, since it can be hard to predict when you’re going to ejaculate and even harder to react in time.

On top of that, your sperm likes to move it.

“Sperm is mobile,” Dr. McDonald-Mosley says. “It swims around. Even if a little bit of semen gets near the woman’s vulva, it can swim into the vagina and increase the risk of pregnancy.” (The vulva is the externally-visible part of a woman’s genitalia.)

What’s more, even if you do fully withdraw before you orgasm, you still release pre-ejaculate during sex. And that might carry a small amount of eager sperm.

Bottom line: Even if you do it perfectly—which is really difficult—withdrawal isn’t foolproof.

“If you aren’t comfortable with that uncertainty, you really should think about other methods that are more reliable,” says Dr. McDonald-Mosley.

Intrauterine devices (IUDs) and birth control implants have low failure rates of less than 1 percent. Talk your partner about whether or not she’s comfortable using one of these more permanent methods.

In comparison, condoms have a 2 percent failure rate with perfect use and an 18 percent failure rate with typical use. Birth control pills have a 0.3 percent failure rate with perfect use and a 9 percent chance with typical use.

Still, if you and your partner are OK with the potential of an unplanned pregnancy and would like to use the pullout method, there are some things you need to know to employ it more effectively.

Performing Kegel exercises can strengthen your pelvic floor muscles, allowing you to gain better control of your ejaculation, says Dr. McDonald-Mosley. With practice, these exercises will help you hold off your ejaculation until you’ve pulled out.

“Contract your pelvic floor muscles—the ones that allow you to stop urinating midstream—for 10 seconds and release them,” she suggests. The more you do these exercises, the stronger the muscles will get.

Practicing them when ejaculation is imminent—like while masturbating or while having sex with a condom—will help you get used to delaying ejaculation until you’ve fully pulled out.

A little knowledge of the female menstrual cycle is important, too. She’s most likely to get pregnant in the middle of it, so use a backup method like condoms during that time, says Rachel Jones, Ph.D., a research scientist at the Guttmacher Institute of sexual and reproductive health.

Even if you use withdrawal perfectly to prevent pregnancy, it still won’t protect you from sexually transmitted diseases. A condom is necessary to cover that base.