I remember the first time I understood that my migraines had a pattern. Four months of tracking in a notes app, and the dates kept clustering around the same point in the month. Always the days just before my period. Nobody had told me this was a thing — not a doctor, not a neurologist, not the pharmacists I had consulted about whether I was taking too much ibuprofen. I had spent years treating each migraine as an isolated event. A failure of hydration. A punishment for the glass of wine. I had a list of suspected causes as long as my arm, and not one of them said: your estrogen just dropped and your trigeminal nerve is reacting. This is that explanation.