No form of birth control is 100 percent effective. Now, a new study provides an explanation for why a small number of women who use hormonal contraceptive methods still become pregnant, even if they use them correctly.

A new study published in Obstetrics & Gynecology explains that some women have an uncommon genetic difference that makes hormonal contraception less effective for them.

In the paper, researchers at the University of Colorado School of Medicine say that around 5 p