Condoms work well to prevent HIV infection when one partner is HIV infected and the other isn't. Studies show that HIV-uninfected partners are 80% less likely to become infected, if they use condoms properly and consistently, compared with those who do not. Consistent condom use means using a condom for every act of vaginal (penis in vagina) or anal (penis in butt) sex.

But sometimes sex gets going before the condom goes on. What are the risks to an HIV-uninfected woman if she and an HIV-infected man start having unprotected sex, but put on a condom before he ejaculates?

Some studies have suggested that HIV-infected men who withdraw before ejaculating may reduce the risk of infecting their partners with HIV compared with those who don't--but the risk is still lower than it is among those who use condoms properly and consistently. Why? Well, we know that pre-ejaculate, the sticky-yet-slippery clear fluid that a man produces after being sexually aroused but before he ejaculates, can have the virus in it, and in enough amounts to be infectious.

A study of recently infected men who have sex with men indicated that the odds of becoming infected with HIV as a result of having anal sex with delayed condom use were about the same as the odds of becoming infected as a result of having unprotected anal sex.

Having unprotected sex also increases the risk of contracting other sexually transmitted diseases (STDs), such as syphilis, gonorrhea, Chlamydia, and genital herpes. Although these infections are treatable and (with the exception of herpes) curable, they often produce no symptoms and can increase the risk of becoming infected with HIV. Using condoms properly and consistently can decrease your risk of contracting these infections as well.

So the bottom line is this: Putting on a condom after you already have started having vaginal or anal sex is more risky than using a condom from the start. If it is difficult to talk about condom use with your partner, then it's time to find someone who can talk with both of you. Many cities have STD clinics and HIV counseling and testing sites that can help partners have these discussions. Your doctor or your partner's doctor may be able to do this, too.