When a man found a pregnancy test in his bathroom cupboard, left there by his former girlfriend, he thought up a terrific jape. He’d take the test himself. But when he had done it, he received quite a shock. That’s because the stick was displaying two solid lines, indicating that he was pregnant.

Indeed, when urine passes over the pregnancy test, it detects whether a certain hormone is present. That substance is beta human chorionic gonadotropin (beta HCG), and it’s most commonly produced by a woman’s placenta as it develops in the womb. It’s rare for there to be any other source of beta HCG, but as it turns out, something else can produce it.

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