The condom acts as a barrier and catches semen, so it does not allow sperm to reach an egg and fertilise it.
When using condoms, it is important that they are put on before any sexual contact with the woman's vagina, as some semen can leak from the penis before ejaculation. This varies from man to man, but if this does happen or semen leaks into the vagina due to a split or incorrectly worn condom then the woman should seek advice on emergency contraception, such as the morning-after pill. This can be obtained from a pharmacy, GP or sexual health clinic. Both partners may also want to consider having an STI and HIV test