How do pregnancy tests work?
In order to determine if you are pregnant, a pregnancy test may provide you with the answer. This test can be done in the privacy of your home using one of several available test kits. The test can also be done in your doctor's office. Home pregnancy tests are always done on urine, while those in a doctor's office or clinic may be done on urine or blood. Home pregnancy tests first became available in 1978.
All pregnancy tests are based upon measurement of the same hormone -- human chorionic gonadotrophin or HCG -- which is only present in a woman when she is pregnant. This hormone is made after the egg is fertilized, and its levels rise rapidly during early pregnancy. It acts to support progesterone, a hormone necessary to maintain the pregnancy.
Most home pregnancy tests are done because of a missed period. If a woman has regular menstrual cycles, a home test can be very informative. Sensitivity of the different home tests kits varies. Some kits may be able to determine whether or not there is a pregnancy three to four days prior to the anticipated onset of days of the menstrual flow.
Manufacturers of home test kits claim that their kits are 97%-99% accurate, but the sensitivities of different test kits depend on the levels of HCG in the urine required for a positive result. If your test result is positive, see your healthcare provider promptly. If your test is negative and you believe you may be pregnant, you should repeat the test again within a few days.
How soon can you take a pregnancy test?
Sometimes a home test is negative if the test is done too early because there may not yet be enough HCG to test positive. Some home test kits come with supplies for two separate tests. Repeat testing is recommended in all such cases.
Urine hCG tests are qualitative because they only detect HCG levels above a certain level. These tests are used when it is enough to know simply whether you are pregnant or not.
By contrast, blood hCG tests are usually quantitative and they can measure the exact level of HCG. These tests are more sensitive and can tell if you are pregnant even before you miss a period (as early as one week after conception).
Your doctor may also want to know the exact amount of HCG in order to evaluate the progression of your pregnancy. This can help determine the age of a fetus and identify problems with a pregnancy. In very rare cases, tumors or other conditions can result in the production of HCG and lead to false positive pregnancy test results. Your doctor can explain more about the significance of HCG levels as they apply to your specific situation.
Taking a medication containing HCG can, not surprisingly, can cause a false positive test result. While alcohol and illegal drugs do not affect pregnancy test results, remember that you should NOT drink alcohol or use illegal drugs if there is any possibility you are or could become pregnant.
Medically reviewed by Wayne Blocker, MD; Board Certified Obstetrics and Gynecology
NIH. The History of the Pregnancy Test Kit.