It can lead to a penile fracture.
Apparently, "woman on top" is the sexual position most likely to lead to a penile fracture, according to a study from the University of Campinas in Brazil.
After culling all instances of this horrifying type of injury over 13 years at three Brazilian hospitals, researchers found that among heterosexual men, 50% of them happened when the female partner was on top.
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The study, published in the journal of Advances in Urology in April of last year (and recently unearthed by New York magazine's The Cut) included a pretty small sample size, only 44 confirmed cases of penile fracture, but to be fair it's a (thankfully) uncommon injury, according to the Urology Care Foundation. In the case of the study, there were only 44 patients in the more than a decade of tracking in Campinas, Brazil, a city with over 3 million residents.
The theory is that “when a woman is on top, she usually controls the movement with her entire body weight landing on the erect penis, not being able to interrupt it when the penis suffers a wrong way penetration.” (Coming in second at 28.6% was "doggy style," which took the top spot for penile fracture danger among homosexual men.)
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But wait: How does a fracture even happen? There’s no actual bone in a boner of course (sorry), but there is the tunica albuginea, which is a sheath of fibrous tissue that's wrapped around the urethra and two other tubes, called corpora cavernosa, that fill with blood during an erection. The tunica albuginea is stretched and very tight when the penis is erect, and a trauma (you know, like the full weight of a grown woman slamming on to it) can cause it to snap, leading to a tear or fracture. The “classical triad” of symptoms: an audible crack (with an instant loss of erection), swelling, and pain. A penile fracture almost always requires surgery to repair, so if it happens get to an ER stat.
Really though, you can file this one under possible but not probable. The Urology Care Foundation says simply being aware that it's possible is the best thing you can do to prevent it. Instead, focus on the many health benefits of a satisfying sex life and feel free to have at it!