Safer sex has two challenges: birth control and death control. All sex involves some risk of becoming pregnant or ill. Consistent use of condoms, or other barriers, drastically reduces the risk of getting a sexually tranmitted illness. Carefully selected and used birth control can dramatically reduce the chance of pregancy. Shouldn't you know the odds of your chosen method before having sex?

Abstinence is free and, when practiced perfectly, 100 percent effective in preventing both disease and pregnancy. In the heat of the moment, however, most people fall short of the mark. We can't all be Catholic clergy, apparently.

Either the withdrawal method (pulling out before coming) or the rhythm method (only having sex at the times of the month you think you cannot get pregnant) result in one in four couples being pregnant after the first year. Flip a coin twice. If it came up heads both times, you're pregnant. Not great odds.

If only using condoms, one in seven couples will be pregnant after the first year—closer to heads coming up three coin flips in a row. Condoms excel at reducing the spread of sexually transmitted diseases. If you enjoy being able to have kids later, having an immune system, or peeing without it burning, using condoms when having sex is probably a good idea. Physical barriers are your best bet for death control.

Only one in 13 couples using oral contraceptive pills ("the pill") end up pregnant after the first year of use, which is closer to flipping four heads in a row. Injected hormone-based birth control—like Depo-Provera or Lunelle—are even better, with about 1 in 33 couples ending up pregnant after the first year. Call that five heads in a row. Neither the pill nor injected hormones protect you against STDs.

The numbers above are for typical use. With practice and training, you can do better. In fact, perfect usage of oral contraceptives can be astonishingly effective, with only 1 in about 330 couples ending up pregnant after a year of use.

Condom broke or slippped off? Didn't quite withdraw in time? Tearfully screaming, "Swim toward the light"? The morning-after pill works remarkably well at preventing pregnancy, up to 120 hours after sex—the sooner, obviously, the better. Jot down 1-888-NOT-2-LATE and put it in your wallet, purse, or cell phone. They'll hook you up.

The best plan for birth/death control is combining two different methods, which multiplies the success rates at preventing pregnancy and gets you death control as well. For example, with typical use of condoms plus the pill you get protection against STDs, and 1 in 80 couples will be pregnant after a year.

All of this doesn't count as medical advice, just a taste of what is known. Go to a professional—a public-health nurse, a family-planning clinic, your doctor—and come up with a plan for you. As my mom, a public-health nurse, says: "Stay healthy. Don't make babies. Have fun."

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