8 in 100 women who have unprotected sex one time during the fertile part of their cycle will become pregnant. If these 100 women take progestin-only ECPs (like Plan B One-Step or Next Choice), about 1 will become pregnant. If 100 women take ECPs with estrogen and progestin, about 2 will become pregnant. The IUD works even better. Only 1 in 1,000 women who have an IUD put in after having unprotected sex will become pregnant.
The sooner you use emergency contraception after unprotected sex, the more likely it will prevent pregnancy. But you must use it correctly. For Plan B One-Step, only one pill is needed but it should be taken within 72 hours of sex. For Next Step, the first pill should be taken within 72 hours of sex and the second pill 12 hours later. (Source WomensHealth.gov
Plan B’s Problem With Heavier Women Isn’t News in Europe (Click to read article). Norlevo is less effective for women weighing 165 pounds or more, and not effective for those weighing above 176 pounds.
An IUD can also be used as Emergency Contraception.
Emergency Contraceptive Pills: How They Work, Safety and Side Effects
The Morning after Pill: What Everyone Should Know in 2015