As my calculus teacher scribbles tangled equations on the chalkboard before me and my fellow juniors, I try to calculate how many days into my period I was when I lost my virginity. Was I in the second week? Maybe day 20?

If the sperm can live for three days inside of me and my egg was traveling at a constant rate toward wherever it goes to be fertilized, could I be pregnant now? It's like an impossible math problem on a final exam, only I have far more at stake than a decent grade.