Identification

• Home pregnancy tests detect the presence of hCG, or human Chorionic Gonadotropin, in the urine. Tests performed at a doctor's office can detect hCG in the urine, or in samples of a woman's blood. Early response pregnancy tests, such as those that claim to be usable up to several days before an expected period, may also detect a variant of hCG called hyperglycosylated hCG or hCG-H to determine the total level of hCG.
  
  

Significance

• When the fertilized egg, or blastocyst, begins implanting itself in the uterus, the part that will eventually become the placenta starts producing hCG-H and hCG. These hormones signal the uterus not to shed its blood-rich lining, so that it can nourish the developing baby and allow it to attach to the womb. This is also what causes a pregnant woman to miss her period. As hCG levels increase, they become significant enough to be detected in the urine or bloodstream of the mother. Since most the hCG in early pregnancy is made up of hCG-H, early pregnancy tests must also be able to detect this variant of the hormone.
  
  

Time Frame

• Implantation and the production of hCG starts to take place about a week after fertilization, but may occur later or sooner in some pregnancies. Until implantation begins, blood or urine tests cannot detect pregnancy. Many early pregnancy tests can start detecting hCG in the urine up to five days before a missed period, and blood tests, which are more sensitive than urine tests, may detect hCG as soon as implantation begins. However, many women's hCG levels may not be high enough to detect with an early home pregnancy test.
  
  

Misconceptions

• Because many early result home pregnancy tests claim to be 99 percent accurate, and claim that women can use them up to five days before a missed period, many people assume this means such tests are 99 percent accurate early on. However, early pregnancy tests often give false negatives. For example, when using a popular brand of early result home pregnancy test, only 58 percent of women who later found out they were pregnant got a positive result five days before their missed period. By the day before an expected period, 94 percent got a positive result. For this reason, women who get a negative result should test again a few days to a week later.
  
  

Considerations

• The total level of hCG in a pregnant woman's body doubles about every 2 days for the first 8 weeks or so. This is why even early pregnancy tests are much more accurate at detecting the hormone later in the pregnancy than they are immediately after implantation. In rare cases, some women may receive a false positive early pregnancy test. However, it's much more common for women to get a false negative early pregnancy test, and not get a positive result until hCG levels increase.