The frogs also developed a side effect that Hogben tried to counteract by injecting the frogs with pituitary extract from an ox. He noticed that female Xenopus frogs ovulated within hours of being injected with the extract. In this way, Hogben serendipitously discovered a human pregnancy test. He knew that the ox extract chemically resembled human chorionic gonadotropin (HCG), a hormone released by pregnant women. He confirmed that female Xenopus frogs, when injected with urine from a pregnant woman, ovulated within hours.