It is placed over the erect penis and is worn for the duration of intercourse. There is a small reservoir at the top of the condom where the man’s semen is held after ejaculation. This prevents the semen entering the woman’s cervix to fertilise an egg. Condoms do not only prevent pregnancy – because they prevent the transfer of bodily fluids, they are effective at preventing the spread of STIs. STIs can be passed from person to person not only through vaginal sex, but also anal and oral sex. No matter what sexual act you are taking part in, it is always a good idea to use a condom.