Home pregnancy tests measure the presence of a telltale hormone in your urine called human chorionic gonadotropin (hCG)

If you think you are pregnant, you may want to test yourself at home with a home pregnancy test. You can buy test kits at a drug store without a prescription. Home use kits measure a hormone called human chorionic gonadotropin (hCG) in your urine. However, these tests are qualitative, the results are either positive or negative for pregnancy. Many home pregnancy tests claim to be 99 percent accurate on the day you miss your period. It most home pregnancy tests don't consistently spot pregnancy this early, the tests are considered reliable when used according to package instructions one week after a missed period. The hormone is released in a pregnant woman's urine.

Home pregnancy tests and how they work

Home pregnancy tests measure the presence of a telltale hormone in your urine called human chorionic gonadotropin (hCG). This hormone, produced by cells from the placenta, first gets into your bloodstream when the fertilized egg implants in your uterus (on average 8 to 10 days after fertilization). The amount of human chorionic gonadotropin (hCG) in your body then increases rapidly over the next few weeks, doubling in amount about every two days.

There are two main types of pregnancy tests. Both are commonly used to detect pregnancy.

Home pregnancy tests are available at your local drugstore or pharmacy and at some large department stores. Blood pregnancy tests are performed by health care professionals in an office or a clinic. Home pregnancy tests are usually about 97% accurate, but this varies from brand to brand. The amount of human chorionic gonadotropin (hCG) in your body then increases rapidly over the next few weeks, doubling in amount about every two days. HCG can also be detected in your blood, around 10 days after you conceive, and your GP may advise you to have a blood test.Pregnancy blood tests are performed health clinic. They detect pregnancy by measuring hCG levels in your blood. The most sensitive test of pregnancy is best performed by a laboratory using a sample of your blood.

Blood tests performed at your clinic have a 99% accuracy rate and can detect pregnancy earlier than most urine tests. Blood tests can be performed 7 days after you ovulate and still provide accurate results. Some urine tests can be used on the first day that your period is due, however, it is unlikely that you will achieve that 97% accuracy rate if you test so early. The quantitative blood test detects pregnancy by measuring the exact amount of hCG in your bloodstream. It can also be used to detect how far along you are in your pregnancy. It does not measure your hCG levels, and only provides a yes or no answer as to whether or not you are pregnant. Home pregnancy tests only check hCG levels, not progesterone. They are completely different hormones.

So how can all these tests claim to be accurate so early in pregnancy?

They don't. They only claim to be "greater than 99 percent accurate" in general, and then separately, they suggest that you can use them as early as the day you miss your period. According to the current Food and Drug Administration (FDA) regulations, in order to claim that a home pregnancy test is "greater than 99 percent accurate," the manufacturer only needs to demonstrate that the test performs as well in the lab as an existing test more than 99 percent of the time.