Pre-cum is one of those things you know exists if you have sex with people with penises.
It appears when the penis-haver is super turned on - for example during foreplay or masturbation, and generally just before they ejaculate.
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The fluid comes from the ‘cowpers glands’ located at the base of the penis. As it passes through the ‘urethra’ (the tube through the penis that people pee from), it mixes with mucus produced by the urethral glands. Finally, it oozes out of the end of the penis and out comes the pre-cum!
Pre-cum acts as a lubricant, to reduce friction during sex. It is lightly salty, which helps to neutralize the otherwise acidic environment in the urethra, caused by urine. This helps the sperm to survive.
Although it doesn’t generally contain much sperm, it’s possible that a few may be left over from the last time the person ejaculated. These may come out with the pre-ejaculate, especially if they haven't passed any urine since then. So always bear in mind that it only takes one sperm to get pregnant!
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Bacteria and viruses can be passed on in this fluid, including HIV.
Women do produce extra fluid during intercourse. This comes from ‘ bartholins glands’ located in the vagina. Additional fluid comes from the vaginal wall itself, and from the cervix. So it’s possible for women to get quite wet during intercourse, giving the impression of a pre-ejaculate.