From the 1920s to the 1960s, a pregnancy test was performed by injecting the woman’s urine into an African clawed frog. If the woman was pregnant, the frog would ovulate and spawned eggs would become visible around its pelvis within a few hours. Millions of these frogs were specifically bred for this test in laboratories around the world.
Modern pregnancy tests are based on the detection of a hormone called human chorionic gonadotrophin (HCG), which is produced in the first few months of pregnancy by the placenta and can be detected in blood or urine as early as 12 days after conception (ie. before a period is even missed). At this early stage, a false negative result is possible, and the tests are more reliable if carried out a couple of days after the missed period. A negative test may mean that the pregnancy is not far enough advanced to be detected, rather than that the woman is not pregnant, while a positive test is almost invariably correct.
The pregnancy test consists of mixing a few drops of the woman’s urine with specific chemicals. If HCG is present, a chemical reaction will take place. In a test carried out in a test tube, the mix of urine and chemicals will form a characteristic deposit; but more often the urine is added to one side of a small flat plastic container and as the urine moves across this it interacts with chemicals that will change colour if the test is positive. To ensure a reliable result, the test is generally carried out 2-7 days after the first missed period (ie. 16-21 days after conception).
A pregnancy test can be carried out at home with a kit purchased from the chemist, but more reliable tests are performed by doctors using a sample of blood.
Although pregnancy actually occurs about two weeks after a woman had her last period, for convenience doctors always date a pregnancy from the first day of that last menstrual period.