It is possible to become pregnant from a man's pre-ejaculate fluid (pre-cum) under certain circumstances, though the chances are quite low. Pre-cum itself does not contain sperm, however sperm left behind from a previous ejaculation might be an eager stow-away on the pre-ejaculate cargo ship.
When some men are sexually aroused, pre-cum may be visible as a clear liquid at the tip of the penis. Pre-cum is an alkaline fluid released by the Cowper's glands, which are located at the top of a man's urethra, just below the prostate. Pre-cum helps neutralize the urethra's acidity, protecting sperm that will pass through the urethra during ejaculation. After ejaculation, whether by masturbation or sex with a partner, sperm may be left over in the urethra, the tube that semen travels through. Urinating between ejaculations flushes the urethra of these leftover sperm and clears the way for pre-cum. If sperm remain in the urethra from a previous ejaculation, they may be released with pre-cum. During unprotected vaginal sex, this could allow sperm to enter the vagina and possibly fertilize an egg, even if the man were to pull out before ejaculating.
Some people practice withdrawal (aka the "pull out" method) as a form of birth control. During vaginal sex, this involves pulling the penis out of the vagina just before ejaculation. Withdrawal may not always prevent pregnancy, in part because of the small possibility of leftover sperm in pre-cum.Men can urinate before having vaginal sex to ensure that their lingering swimmers wash away; of course, when using withdrawal, the male partner still has to pull out before ejaculating to prevent pregnancy.
Even if pregnancy from pre-cum is not an immediate concern, the risk of a sexually transmitted infection (STI) is. Using condoms can help reduce the risk of STI transmission as well as pregnancy.