How does birth control regulate your period?

I thought it was supposed to stop you getting pregnant, so then it should stop your period altogether! Why do doctors prescribe this to girls who have irregular periods.How does birth control regulate your period?
The hormones present in the pill (oestrogen and progesterone) prevent you from ovulating. Obviously, if no egg is released each month, there is no chance that you can be impregnated. It essentially tricks the body into behaving as it would if you were pregnant. The period which you get at the end of each pill packet, during your pill-free week, is not a natural period but is rather what is known as a 'false period', and is simply the body's reaction to the lack of hormones being introduced during that week. It is essentially not necessary to have a pill-free week in order to induce a false bleed, but is thought to provide comfort to women who are on the pill - it can be quite disconcerting to never have a period at all, as it is usually the best indication that you aren't pregnant.