The cervix is the "mouth of the uterus" that projects into the vaginal vault. The cervix normally thins out and dilates during labor to allow a fetus (baby) to be delivered vaginally.
Cervical ectopic pregnancies are particularly dangerous since the (main) uterine artery approaches the uterus at the level of the cervix. Therefore, surgical removal of a cervical ectopic pregnancy may result in sudden massive bleeding and the need to perform an emergency hysterectomy (removal of the uterus). A trial of medical management (such as methotrexate), predominantly in a hospital setting, is often considered for these pregnancies.
Implantation into the cervix is (thankfully) rare.