How does birth control prevent pregnancies if you still get your period once a month?

Artificial 'period': They prevent ovulation from occurring by exposing the body to higher than normal hormone levels (they 'simulate' a pregnancy state). The last 2-7 days of the pill pack (depending on which pill you are taking) are placebos, and the lack of hormones in those pills will cause your body to have a progestin-withdrawal bleed (the "period" at the end of the pill pack). You are not ovulating when taking.