In an ectopic pregnancy, the embryo has implanted anywhere other than the uterus. HCG levels will rise slower than normal or plateau. Ectopic pregnancies are dangerous and need the treatment of an OB/GYN.
In an ectopic pregnancy, generally women experience even higher than normal amounts of HCG during the early weeks. This does not mean if you have high HCG you definately have an ectopic pregnancy.