Ectopic pregnancy can occur when emergency contraceptive pills are used to prevent pregnancy. The pill works by delaying or inhibiting the ovulation function of the ovaries, which prevents the ovaries from emitting eggs normally, or by altering the mucus of the cervix, which prevents the male sperm from passing through the cervix and inhibits the formation of a fertilized egg. Another way of action is to change the properties of the endometrium so that the fertilized egg does not settle and develop. If the emergency contraceptive pill is taken when a fertilized egg has already formed, an ectopic pregnancy can occur if the fertilized egg does not reach the uterine cavity but instead lays in the fallopian tube. If there is no obvious withdrawal bleeding 7-10 days after taking the emergency contraceptive pill, a blood HCG test should be performed promptly to rule out the possibility of early pregnancy, and if it suggests early pregnancy, ectopic pregnancy needs to be ruled out as soon as possible.