How it prevents pregnancyThe pill prevents the ovaries from releasing an egg each month (ovulation). It also:thickens the mucus in the neck of the womb, so it is harder for sperm to penetrate the womb and reach an eggthins the lining of the womb, so there is less chance of a fertilised egg implanting into the womb and being able to growThe pill is over 99% effective if used correctly.Starting ...