I know of a couple of people falling pregnant using withdrawel - overall it is around 70% effective as a contraceptive method, and much more effective if you use extra back-up during your fertile window (eg. condoms/diaphragm/abstinence).

It's a pretty easy method to mess up - perceptions change dramatically in the heat of the moment, and there only needs to be a tiny slip-up! I usually advise people to use condoms instead of withdrawel in their possibly fertile times... unless they are open to a new addition to the family

Charting fertile mucous and temperature is over 97% effective as a form of natural contraception when done correctly.