The average American woman weighs 166 pounds. New data suggests that the Plan B morning-after pill is less effective if you weigh more that 165 pounds, and won’t work at all for women who weigh more than 175. What’s more, writes Kate Clancy (an anthropologist who studies women’s reproductive issues), the dosages for regular old daily birth control are set for average-to-low-weight women. If your BMI is over 25, the pill won’t work as well for you.