> I can't find any statistics at the moment, but I believe the
chances are extremely small with either of the two pills and
almost nonexistant with the combination pill that an egg
could be fertilized and then prevented from implanting.

Please keep in mind: that assumes perfect use. Humans are
imperfect. I read recently (no source, sorry, can't remember
where) that most unwanted pregnancies that happened
when on the pill were due to the woman not taking the pill
at a consistent time each day.

Luisa