Some morning-after pills contain only one hormone, levonorgestrel (Plan B, Plan B One-Step),
and others contain two, progestin
and estrogen. Progestin prevents the sperm from reaching the egg
and keeps a fertilized egg from attaching to the wall of the uterus (implantation). Estrogen stops the ovaries from releasing eggs (ovulation) that can be fertilized by sperm.
The morning-after pill is designed to be taken within 72 hours of intercourse with a second dose taken 12 hours later.