Hogben never alluded to this application in his early reports, but it wasn’t long before he was working towards it. Disenchanted with racism in South Africa, he returned to Britain shortly after his seminal experiments and brought a colony of Xenopus with him. His colleague Charles Bellerby worked out how to raise the frogs appropriately, showed that they would reliably lay eggs when exposed to the urine of pregnant women, and confirmed that when they're not mating, they don't lay eggs spontaneously. Another team from South Africa had been doing similar work, and as academics are wont to do, the two groups developed a bitter feud. It was never truly resolved, although Hogben could be said to have won, since the test that resulted from this work took his name.

The “Hogben test” was simple. Collect a woman’s urine and inject it, fresh and untreated, under the skin of a female Xenopus. Then, wait. If the woman is pregnant, between five and 12 hours later, the frog will produce a cluster of millimeter-sized, black-and-white spheres. The results were reliable. One researcher reported that after injecting 150 frogs, he never got any false positives and only missed three actual pregnancies. And as one doctor wrote to Hogben’s colleagues: “Thank you for your report on the pregnancy test on Mrs. X. You may be interested to know that of one GP of many years’ standing, one specialist gynaecologist and one frog, only the frog was correct.’”

There’s a long history of folk pregnancy tests: As my former colleague Cari Romm once wrote, “a long, long time before women peed on sticks, they peed on plenty of other things.” But the first reliable test was created in 1927 by German scientists Bernhard Zondek and Selmar Aschheim. Their “A-Z test” involved injecting human urine into immature female mice for a few days, dissecting the animals, and checking if their ovaries were bigger than normal.

A later version of this test used rabbits instead of mice. For some reason, people came to believe that the rabbits would die if the test was positive, and the phrase “the rabbit died” became a euphemism for pregnancy. In truth, the rabbits always died, because, like the mice, they had to be dissected to check the size of their ovaries. This meant that testing for pregnancy was laborious, expensive, and bloody work. In one Pregnancy Diagnosis Station, run by a friend of Hogben’s, around 6,000 rabbits were sacrificed every year.

By comparison, the Xenopus test was faster, simpler for practitioners, and much easier on the animals. The frogs can live for up to 30 years in captivity, and since the tests didn’t kill them, they could be reused. They were also easy to get. At first, doctors imported them from South Africa where, in the words of scientist Edward Elkan, “animal dealers seem to have no difficulty in catching as many as are required.” Later, many Xenopus colonies were established overseas.