An ectopic pregnancy occurs when a fertilized egg implants itself outside of the uterus. Ectopic pregnancies usually occur in a fallopian tube (called a tubal pregnancy). Occasionally, the egg may lodge itself in the ovary, and more rarely, in the cervical canal or the abdominal or pelvic cavities. The fertilized egg doesn't usually grow into a recognizable embryo and can't be transplanted into the uterus.
In a normal pregnancy, the egg is fertilized in the fallopian tube. Little hairs in the fallopian tube move the egg down to the uterus, where it implants itself. If there's scar tissue in the fallopian tube, or it's blocked for some other reason, the fetus will begin to grow outside of the uterus. A woman who has an ectopic pregnancy must have the pregnancy removed because the fetus can't develop properly outside of the uterus and it is dangerous to the woman's health.
Although they're becoming more common in recent years, ectopic pregnancies are generally rare. About 2% of pregnant women will have an ectopic pregnancy.