Ceepea
Well-Known Member
This restricts the follicles from growing, and by extension, an egg from growing and releasing from the ovary. Essentially, these synthetic hormones trick the ovary into thinking that it's already released an egg. The endometrium still builds in the uterus and is released, but this is known as a "withdrawal" period. It's the body's reaction to the withdrawal of the normal hormonal cycle. This is why a period while on the pill is usually lighter and shorter; the corpus luteum can't grow unless the ovary has released an egg. The progestin may also make it harder for sperm to enter the fallopian tube by making the vaginal mucus thicker.
http://health.howstuffworks.com/sexual-health/contraception/birth-control-pill2.htm