But as strange as it sounds to modern ears, “piss prophecy,” so to speak, never really fell out of favor—at least not with pregnancy tests. The name changed, and the tools changed. And what also changed, most significantly, was who got to play the part of the prophet.

Doctors in the 18th and 19th centuries, shaped by the scientific discourse of the Enlightenment, abandoned the belief that urine could simply be eyeballed, instead pursuing the idea that it must contain some less easily identifiable traits—some bacteria or crystal structure, visible only under a microscope—that could signify a pregnancy. Around the end of the 19th century and the beginning of the 20th, scientists began to discover the chemicals that regulated various functions in the human body, including reproduction. The word “hormone” was coined in 1905; in the 1920s, human chorionic gonadotropin (hCG)—a hormone found in high concentrations in pregnant women—was identified.

The first true precursor to today’s pregnancy test was developed in 1927, when the German scientists Selmar Aschheim and Bernhard Zondek discovered that injecting a pregnant woman’s urine into a mouse or rat would send it into heat, which could be ascertained only by dissecting the animal. Over the next few decades, the AZ test, named after its creators, replaced rodents with rabbits—the phrase “the rabbit died” was, at one point, a euphemism for a positive pregnancy test—and then frogs (so many frogs were exported from southern Africa to the U.S. for pregnancy tests, in fact, that some scientists believe they may be the source of a fungal disease currently threatening the country’s amphibian population). In the 1960s, scientists ditched the animals entirely, turning instead to immunoassays, or tests that combined hCG, hCG antibodies, and urine—if a woman was pregnant, the mixture would clump together in certain distinctive ways.

While women no longer needed a frog or a rabbit, though, they still needed a doctor. The test also frequently turned up false positives, as hCG could easily be confused for other similar hormones. A more accurate test wouldn’t arrive until 1972, when Judith Vaitukaitis and Glenn Braunstein, researchers at the National Institutes of Health, identified an immunoassay that could successfully measure levels of hCG, rather than simply detecting its presence. Recognizing the potential of their discovery, the two attempted to patent it on behalf of NIH—but were shot down by the institute’s lawyers, who argued that because the project had been funded with public dollars, nobody should receive any royalties for the resulting product. Instead, the knowledge went immediately into the public domain, until pharmaceutical companies recognized the same potential that Vaitukaitis and Braunstein had seen.

In the meantime, both scientists moved on to other things—which, according to an oral history by NIH, may have been an act of self-preservation as much as anything. To obtain enough urine for another part of their hormone research, unrelated to pregnancy tests, the scientists had struck up a deal with a nearby retirement home for nuns: The nuns would collect their urine over several weeks in plastic containers big enough to hold around 15 gallons at a time. Once a month, Vaitukaitis, Braunstein, and their colleagues—including “someone who was strong enough to lift these bottles,” Vaitukaitis recalled—would trek over to haul it all back to the lab.