"Every morning when she wakes up, Becca, a college student in Pennsylvania, puts a teardrop-shaped thermometer called the Daysy under her tongue. If it lights up green, she knows that day she and her boyfriend can have sex without a condom. If it’s red or yellow, they need to use protection."
http://www.theatlantic.com/health/ar...method/383545/
Stumbled across this article yesterday. It always amazed me that a lot of women avoid birth control, including providers (especially younger ones).