Plan B
"My experience is different from many of the people posting here. I've never been that good about tracking when I'm fertile, but I do have very regular periods. Several months ago, I had unprotected sex a few days after my period ended and took Plan B within 72 hours. About a week before my expected period, I had very light spotting for maybe an hour. Then it stopped. Two weeks later, I still had not gotten my period, so I took a test, and it was positive. Please realize that while Plan B is highly effective (I'd used it with success before and am wholeheartedly in favor of it being available to everyone on demand), it's not foolproof and shouldn't replace contraception."