Why women can get pregnant even if they are on birth control pills

We've all heard pregnancy "horror" stories, of women who thought they were protected by birth control pills, but still got knocked up anyway.

According to the Centers for Disease Control and Prevention, 17.1% of women in the US between the ages of 15 and 44 are currently using the pill as their primary means of contraception.

If used correctly, birth control pills can be very effective, with less than one out of 100 women getting pregnant each year if they always take the pill as directed, Planned Parenthood has stated.

But if the pill is not taken properly, as many as nine out of 100 women could get pregnant each year.

So why is it that some women still get pregnant if they take the pill? Dr. Lubna Pal, an OB/GYN at Yale University, told INSIDER that there are four main reasons.