If you have kinking, scarring, damage, or other abnormality of a Fallopian tube. This is because a fertilised egg (ovum) may become stuck in the tube more easily.

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

If you have kinking, scarring, damage, or other abnormality of a Fallopian tube. This is because a fertilised egg (ovum) may become stuck in the tube more easily. For example:
If you have had a previous infection of the womb (uterus) or Fallopian tube (pelvic inflammatory disease). This is most commonly due to either chlamydia orgonorrhoea. These infections can lead to some scarring of the Fallopian tubes. Chlamydia and gonorrhoea are common causes of pelvic infection.