Although sterilisation is a very effective method of contraception, if a pregnancy does occur, about 1 in 20 is ectopic.

http://www.patient.co.uk/health/ectopic-pregnancy-leaflet

Previous sterilisation operation. Although sterilisation is a very effective method of contraception, if a pregnancy does occur, about 1 in 20 is ectopic.
Any previous surgery to a Fallopian tube or nearby structures.
If you have a condition of the uterus and surrounding area (endometriosis).