"When birth control came out in the 70's, the estrogen levels were so high ovulation never happened. The women never released an egg. They found out about 10 to 15 years later, after women started dying that the estrogen level was too high. They stopped adding so much estrogen, but that also left a higher risk of releasing an egg or a higher risk of pregnancy. To counter the risk of ovulation or pregnancy they started adding progestin. Progestin does not prevent ovulation, so what does it do?
"Birth Control with Estrogen and Progestin work together like this: