An ectopic pregnancy is one that occurs outside of the uterus. A pregnancy occurs when there is a successful fusion of a male pronucleus with a female pronucleus. After a successful fertilization the egg should travel down into the uterus where it implants into the stratum functionalis of the endometrium of the uterus. From here the fertilized egg starts to grow and develop. In an ectopic pregnancy the implantation does not occur in the uterus. Ectopic pregnancies occur in about one percent of all pregnancies.
In the following video an animation of the track a fertilized egg takes, and how this can result in a tubal ectopic pregnancy is shown.
Video 1: Animation of Ectopic Pregnancies