When people say that contraceptives, like condoms, have a twelve percent failure rate, do they mean that they result in pregnancy in twelve out of every 100 women who use them? I hope that means that twelve percent of people who use them become pregnant in a year, or something like that. Otherwise, it sounds as risky as Russian roulette to rely on condoms.
Good news — your second interpretation of contraceptive effectiveness rates is more like it. During a year of typical condom use, between 10 and 15 out of 100 sexually active women will become pregnant. During a year of perfect condom use, that number drops to between two and three out of 100 sexually active women becoming pregnant. Just for the record, 21 percent (typical use) and 5 percent (perfect use) of women who use the female condom experience an unintended pregnancy within the first year of use.
Here's the difference between perfect use and typical use. Perfect use means using a condom during intercourse consistently and correctly every single time, and reflects the effectiveness of condoms themselves. Typical use gets at the reality that people may use condoms incorrectly or may not use them every single time they have sex. That is, the "typical use" condom effectiveness rates you see include the possibility of human error or omission. It follows that typical use condom effectiveness would be lower than perfect use — if someone uses a condom 90 percent of the times they have sexual intercourse there is a higher chance of pregnancy than if they use a condom 100 percent of the time.
As long as we're on the subject of effectiveness, it should be noted that condoms are also highly effective in preventing transmission of HIV and a number of other STIs (sexually transmitted infections). Studies done on heterosexual sero-discordant couples — where one partner is HIV-positive and the other HIV-negative — show that HIV was transmitted in zero to two percent of couples who correctly and consistently used male condoms for both vaginal and anal sex. With typical use, the HIV transmission rate increased to between 10 and 15 percent. While condoms can also reduce the risk of other STIs, but their exact effectiveness is harder to determine.
Practice is the best way to learn to use condoms correctly. For tips for using condoms consistently and correctly, check out How to use a condom properly — Avoid breakage and slippage! in the Go Ask Alice! archive. You or your partner may also want to try using condoms during (mutual or solo) masturbation — there'll be less pressure and anxiety than during sex, so it may be easier to get the hang of the steps. For more tips on condom use, check out the Related Q&As.Alice!