A condom prevents pregnancy by capturing the sperm cells in the ejaculated male semen or “cum.” This prevents the sperm from reaching the egg in the female fallopian tubes and fertilizing the egg.
If a condom falls off or breaks inside the woman, then the sperm can quickly travel into her uterus and tubes and the protection from pregnancy is lost. A small drop of semen contains thousands to millions of sperm cells. This means even the tiny drop of pre-ejaculate found on the tip of an aroused male penis is enough to cause a pregnancy if it gets into the female vagina.
Condoms work best to prevent pregnancy if the man puts it on his penis before he gets anywhere near his partner’s vagina and if he removes his penis from the vagina as soon as he orgasms and ejaculates so that no sperm cells can escape.