So birth control pills contain Progesterone, a synthetic hormone that's part of the whole sneaky trick your body into thinking it's pregnant - it's non synthetic, naturally manufactured version present during pregnancy, progesterone is "the pregnancy hormon". The reason pregnant women create it is because of the fetus's having 50% of someone else's genetic material. It works as a natural immunosuppressor so that the mother's immune system does not attack the fetus. Obviously the mothers who didn't do this didn't have fetuses that made it to term and therefore did not pass on there genetic material.
The reason why women can have this high level of immuosuppressor is because they only have to do it for 9 months at a time. During this time, the have "greatly increased risk of blood clotting, high blood pressure, cancer, mental disturbances, weight gain, hormone swings and immune suppression" (Sarah Myhill, MD).
Think about doing that to your body for 5-10 years - I have plenty of friends that have taken the pill continually for that amount of time.
This causes a bit of frustration for me, because I would like to be able to use some sort of birth control beyond condoms. I just can't bring myself to go through what I did when I was on the Pill and now, after being exposed to the papers in the medical journals that I have access to at work, including the work by Dr. Ellen C. Grant, I am even more terrified of them.