Think you can't get pregnant if your partner pulls out before he comes? Think again. According to the World Health Organization, withdrawal is the most commonly used birth control method around the world, but that doesn't mean it's the most effective. Withdrawal has shortcomings both as a contraceptive and as protection against sexually transmitted infections (STIs) like HIV.

Yes, it's true, when employed correctly, withdrawal can prevent pregnancy. And when other methods are unavailable, that makes it significantly better than no method at all. But it's more complicated than just pulling and praying, as the saying goes.

From the man's side

First of all, a man must be experienced enough to know how and when to properly withdraw his penis during intercourse. Sperm are sly swimmers and strong warriors (they can live up to five days or more in the female reproductive tract). Also, some young men who ejaculate very quickly might not yet have a good sense of what's called their "ejaculatory inevitability"-- the point as they approach orgasm when they just can't stop themselves. "These men don't understand the signals and think they can stay in a little longer, and that's very dangerous," explains Jon Knowles, director of public information for Planned Parenthood Federation of America.

But what if you have good control? Can pre-ejaculate--the small amount of fluid secreted before the man ejaculates--still contain enough sperm to cause pregnancy?

Pre-ejaculate, a lubricating secretion, does not contain sperm when it's produced in the Cowper's glands. Nonetheless, it's theoretically possible for pre-ejaculate to contain sperm as it leaves the penis. Here's how this might happen: If a man had climaxed previously, sperm might be left over from this prior ejaculation and be ushered out of the penis in the next pre-ejaculate. "The counts of sperm in pre-cum that would be picked up this way would be low," says Knowles, "unless the guy and girl are really fertile."

From the woman's side

Some folks feel that withdrawal is an effective contraceptive method if you practice it at a less "risky" time of the month. But how can you be sure?

Although medical science is able to generalize about what time of month women are most likely to get pregnant, for any given woman, predicting her own cycle is far more complex. Every woman's cycle is different, and she must be well in tune with her own unique menstrual cycle and ovulation pattern to predict her chances of getting pregnant. Methods such as fertility awareness, in which a woman charts her daily temperature and consistency of cervical mucous (sometimes known as discharge), require diligence and care. Experts recommend six months of practice before a woman can depend on it for contraception. (If you're interested, contact your local Planned Parenthood organization to link up with a class or teacher near you).

Nineteen percent failure rate

Of every 100 women whose partners practice withdrawal perfectly (meaning, the guy always withdraws on time without any failure), four will become pregnant during the first year. But of every 100 women whose partners practice withdrawal typically (meaning, most of the time he withdraws but sometimes he may not withdraw in time), 19 will become pregnant during the first year.

Sexually transmitted infections

Protection from STIs is another matter altogether. Some STIs are spread through semen and other fluids, and ejaculation may bring increased risk. But even without ejaculation there is significant risk because many STIs can be spread by skin-to-skin contact or other secretions, including vaginal fluids and pre-ejaculate.

With no barrier at all, there is little or no protection. The folks at Planned Parenthood recommend that you understand what the risks are and then decide what level of risk you want to take. "For example, oral sex play generally has a lower risk than vaginal or anal intercourse," adds Knowles. "Protected oral sex is even lower-risk."

All the more reason to think carefully about whether or not you are experienced enough to use withdrawal. Experts at Planned Parenthood recommend withdrawal solely for couples who have established great self-control, experience, and trust with one another. This usually assumes monogamy, the ability to talk about anything, a shared responsibility for the birth control method, communication about the risk for STIs, and comfort with sexual and anatomical language. You can also practice withdrawal while continuing to use another non-barrier method until you're sure you have the withdrawal method under control.

Bottom line: For casual partners, withdrawal is not a casual thing. It is still unprotected sex, and that is the highest category of risk.