History

• The earliest pregnancy tests utilized animals. Injection of mice and rabbits with a pregnant woman's urine would induce ovulation. Unfortunately, the only way to tell whether this was the case was to dissect the animal. In the 1950s, frogs began to be used as indicators of human pregnancy. Frogs of the family Pipidae were induced to lay eggs when injected with the urine from a pregnant woman.
  
  

Function

• Modern tests rely on the concentration of the hormone human chorionic gonadotropin (hCG) circulating in a woman's bloodstream. This hormone is secreted from a developing embryo after implantation (which occurs anywhere from six to 12 days after ovulation). When applied to a home pregnancy test, hCG will show up and indicate a positive test result.
  
  

Time Frame

• When a pregnancy test is taken too soon, levels of hCG are not high enough to register on the test, and a false negative results. Today, the most sensitive pregnancy tests will register positive as soon as eight days after ovulation. There are brands of home pregnancy tests that are more sensitive than others (see link in "Additional Resources" for a list of brands).
  
  

Considerations

• Home pregnancy tests are designed to detect levels of hCG that range from 20 to 100 uL hCG /mL. These tests detect hCG in the urine, so they must be placed in urine (in a cup, or in a urine stream). While concentrations of hCG must be 20-100 uL/mL for a home pregnancy test to work, blood tests can detect hCG levels as low as 1 uL/mL and are thus able to detect a pregnancy earlier.
  
  

Warning

• If a home pregnancy test indicates that the user is not pregnant but the woman's period is delayed, she may want to wait a few days to retake the test. Ovulation can vary from month to month, and chances are the levels of hCG in your bloodstream simply were not great enough to register on the test. Seek medical attention if your period does not begin and you have taken multiple home pregnancy tests.