What is an Ectopic Pregnancy?
An ectopic pregnancy occurs when implantation occurs outside the womb. Implantation can occur in the ovary, the abdomen, the cervix, at the join between the tube and the womb, but the most common place is in the fallopian tube. Pregnancy can even occur in both the womb and the tube at the same time (heterotopic pregnancy), but this is very rare.

How common is it?
The percentage of pregnancies which are ectopic is between 0.25 and 1%

What causes an it?
An ectopic pregnancy is usually caused by conditions which slows down or obstructs the movement of the egg down the fallopian tube into the uterus. Any condition which may have damaged the fallopian tubes increases the risk of a tubal pregnancy. The possible causes of ectopic pregnancies include:

Pelvic inflammatory disease - an infection of the female reproductive organs which can cause scarring of the organs.
A history of Endometriosis - a disease where tissue usually found in the uterus develops in other areas of the body.
Previous tubal surgery
Multiple induced abortions
Pelvic adhesions - bands of scar tissue that constrict the tube