However, overall risk reduction depends on the following: The woman's risk of pregnancy without emergency contraception The time in the menstrual cycle that emergency contraception is given The woman's body mass index (BMI; levonorgestrel emergency contraception is less effective than ulipristal acetate in obese women with a BMI > 30 kg / m2) In the US, oral levonorgestrel emergency contraception is available behind pharmacy counters without a prescription.