Ahaha, yeah. Periods are reliable up until they're not.

Mine was like clockwork all the way up through college. Then when I started working long hours at a stressful job where I was frequently surrounded by pregnant women...

I went on the pill simply to get my body back on a schedule, because I'm an extremely paranoid person and freaked out when I ended up being two weeks late. And I've met many women who weren't nearly so cautious about their bodies: "Of course pulling out is an effective birth control method, I've only gotten pregnant once that way!"