uterine contractions—minor peristaltic versions of which are happening all the time, not just during orgasms—have been shown to reverse direction over the course of a woman’s menstrual cycle. Around ovulation, when a woman is most fertile, they pull material in toward the uterus; during menstruation they expel it. (The reproductive system is smarter than you think, and utterly goal-directed. Not only do sex hormones orchestrate the direction of your uterine contractions, they dilate only the fallopian tube that contains the ovum, so that more semen ends up on that side. They even oversee the
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