When I was in college, a (male, obviously) PhD biology student wrote an op-ed in the school paper arguing that birth control was unnecessary, because women "always know" when they are ovulating due to the "clear and present" physical signs.
This was two decades ago, and I still remember that phrase, because we started calling pretty much anything -- pimples, sneezing, bad hair, hangover -- a "clear and present sign" of ovulation.