For most animals, sex time is obvious. During the fertile phase of their reproductive cycles, females go into heat. They act, smell and look different, sending an unambiguous signal to males: “Come impregnate me.”

But what about humans? Women have sex throughout their menstrual cycles, and don’t show conspicuous outward changes around ovulation — the time of month when pregnancy can occur.

Yet many researchers think there are subtle clues that a lady is in her fertile window. Not quite