Is it normal (or even possilble) to get pregnant after 8 yrs of trying

Heres the deal...my wife and i are both in our early 30's, we have 5 kids, our youngest being 8 years old. Since the birth of our youngest child we have had intercourse maybe 2 to 3 times a week, sometimes more. I've never "pulled out" of her, and she has never gotten pregnant again till now. I just kinda figured i had a low sperm count or something. Now, my question is, can she, all of a sudden, get pregnant after 8 years of not "pulling out". Did we just get lucky or do you think foul play on her part was involved o.O (affair maybe?) PLEASE HELP p.s. we havent changed our daily routines,no fertility pills, no special fertility drinks, no nothing.