A method of pregnancy prevention could cause brain tumors. New Study Links Common Hormonal Contraceptives to Rare Brain TumorsA flawed study in the late 1990's scared millions of women away from hormone replacement therapy. Since that time, we have learned more about how hormones should be administered whether it be birth control or HRT. Top Eight Reasons Your Birth Control Can Fail We all know that birth control is never 100% effective, but it is pretty darn close, right? Here are the top causes for unexpected birth control failure.