Sometimes the truth hurts, but nothing hits harder than a blow to your manhood: All guys start out, in utero, as females.

“Everyone comes from a common genetic and developmental framework that is tweaked by sex hormones,” says Richard Bribiescas, Ph.D., director of the Yale Reproductive Ecology Laboratory. We all start as a generic embryo. You have a set of male or female sex chromosomes, but the distinction doesn’t kick in until your hormones enter the picture, he explains. Without hormones like testosterone, you would stay on the path to womanhood. And, sorry to say, your body already started developing by the time this decision was made—which means your lady parts were already starting to form.

Sound crazy? We’ve got proof: Here are three visible signs that all men started out as women.

YOUR NIPPLES

Nipples are really just chest ornaments for men, but with the right hormones you could be a milk machine, too. Bribiescas says nipples are formed very early during embryotic development. Post-pregnancy, women produce hormones like prolactin and oxytocin that allow their nipples to fulfill their purpose—to produce milk. “Without production of these hormones, men don’t lactate, although they definitely could in the presence of these important hormones,” Bribiescas adds.

YOUR PENIS

During development, various hormones—including Müllerian Inhibiting Substance (MIS) and dihydrotestosterone (DHT)—are produced by males, causing the internal and external genitalia to develop differently than in females, Bribiescas explains. Because of DHT, the genital bud grows into the penis, but without this hormone, it would become the female clitoris. Meanwhile, MIS keeps the Müllerian ducts from progressing any further in men, keeping you from developing the female internal reproductive tract. So yes, that’s right: Your mountain is just a grown-up version of her valley.

THAT WEIRD SEAM RUNNING DOWN YOUR SCROTUM

What is that thing?!? Technically, it’s called the raphe line, and without it, you’d have a vagina. During embryotic development, everyone has an opening at their genitals. “Without male hormones, the opening remains and contributes to the development of the labia and vagina,” Bribiescas explains. When male hormones enter the picture, though, the tissue is fused together, leaving a scar that runs from your anus, over your scrotum and up the penis. Consider it a reminder of what could’ve been.

Rachael Schultz Rachael Schultz is a freelance writer who focuses primarily on why our bodies and brains work the way they do, and how we can optimize both (without losing our sanity).

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