During sexual intercourse, an erect penis is inserted into the vagina. During orgasm, the penis releases semen, which contains sperm, that start to swim up from the vagina, through the cervix, into the uterus and into the upper parts of the fallopian tubes. Not all the sperm make this journey and are flushed out of the vagina. However, some survive for up to five days. It only takes one strong swimmer to fertilise an egg. When this happens, only the head penetrates the egg, as the long tail of the sperm breaks off and a chemical change in the egg stops other sperm from getting in. Unlike women who have a set number of eggs, men produce sperm throughout their lifetime.