According to a large-scale study conducted by leading sexual health researchers, as reported in the Guardian, one in every three babies conceived is by accident. Many of these pregnancies resulted from condoms breaking and because women missed taking their birth control pills.
This leads us to the the number one reason that accounts for the failure of birth control...
#1 Reason for Contraceptive Failure - User Error:
User error varies depending on which method of contraception is used. Birth control must be used consistently and according to instructions in order to attain maximum effectiveness. Certain methods are more prone to user error than others.
According to Professor Glasier, one of the reports authors, "a lot of women aren't terribly good at using contraception, the problem mainly being with the pill and condoms. There's something in human nature which makes us not brilliant at taking pills every day.