Some 24 years ago, Diana Bianchi peered into a microscope at a piece of human thyroid and saw something that instantly gave her goosebumps. The sample had come from a woman who was chromosomally XX. But through the lens, Bianchi saw the unmistakable glimmer of Y chromosomes—dozens and dozens of them. “Clearly,” Bianchi told me, “part of her thyroid was entirely male.”

The reason, Bianchi suspected, was pregnancy. Years ago, the patient had carried a male embryo, whose cells had at some point wandered out of the womb.

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