Most laboratory tests for pregnancy work on the same principle: detection of a key hormone, chorionic gonadotrophin, found in the urine of a pregnant woman. A concentrate of a urine specimen is injected into a test animal (frog, rabbit, mouse); if present, the hormone will cause a recognizable reaction in the sex glands. If there is no reaction, the patient is presumed not pregnant. But no method is foolproof—frogs injected only with distilled water have developed reactions. Some tests (e.g., the Friedman test using rabbits, the Aschheim-Zondek test with mice) are highly accurate, but require 48 to 98 hours for definite...
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