Pregnancy tests are now simple enough that women can quickly and reliably find out whether they are pregnant in their doctor's office or at home.

How Do Pregnancy Tests Work?

All pregnancy tests (whether they use blood or urine) detect a hormone produced by the placenta called beta human chorionic gonadotropin (beta-hCG). This hormone is produced only by the placenta and hence is specific to pregnant women. The test results are not perfect-rarely a woman who is pregnant may produce a negative test result or a woman who is not pregnant may produce a positive test result.

How Soon After I Get Pregnant Do Pregnancy Tests Work?

Most home pregnancy tests can detect pregnancy as early as the fourth day after a missed menstrual period. Blood tests can detect pregnancy as early as three to five days before your period is due.

The placenta must be established before a pregnancy can be detected. The placenta forms after the fertilized egg has traveled from the fallopian tube and has implanted in the uterine lining. Once implantation has occurred, beta-hCG is produced and begins to enter the mother's bloodstream. Thus beta-hCG can be detected in the blood as early as three to five days before a period is due. Beta-hCG is excreted from the mother's body through her urine (after it has been filtered through the kidneys). The hormone gets diluted in the urine and therefore urine pregnancy tests are not positive until after the period is due (when levels of beta-hCG are higher).

Should I Test for Pregnancy at Home or Go to the Doctor?

Whether you decide to go to your doctor or buy a kit from a local drug store is a personal decision. To make sure that a home test is accurate, consider the following steps:

  • carefully read the directions before starting the test and follow the directions closely;
  • you can do the test any time of day-it is not necessary to test in the morning;
  • urinate into a small clean cup (it does not need to be sterile);
  • it is especially important to use the dropper that is provided with the kit and to use only the number of drops of urine specified; and
  • be sure to wait as directed.

If the kit is not clearly positive, you may want to consider repeating the test in a few days or seeing your health care provider.

Do I Need a Blood Test for Pregnancy?

Once a period has been missed, a urine test is just as reliable as a blood test in determining whether a woman is pregnant. In most cases, a urine test is faster and more efficient in confirming a pregnancy. In some cases, a blood test for pregnancy will be ordered. Blood tests are useful when problems are suspected, such as a pregnancy outside the uterus (also known as an ectopic pregnancy) or miscarriage. Blood tests also test for the beta-hCG hormone but, unlike the urine tests, are able to measure levels of beta-hCG. Repeated blood tests may be useful in diagnosing bleeding problems or a possible ectopic pregnancy. Generally, your doctor will need to analyze the change in beta-hCG over a couple of days; therefore, repeated blood tests are required. If the pregnancy is going well, levels of beta-hCG should double about every two days.

When Is Ultrasound Useful in Testing for Pregnancy?

Ultrasound can be used to gauge the progress of pregnancy at six weeks after the last menstrual period. By this time, an embryo should be seen in the uterus. While the embryo is present before this time, it cannot be detected with current ultrasound machines. After eight weeks of pregnancy, the beta-hCG hormone is produced at a constant rate. At this point, it is best to use ultrasound to monitor progression of the pregnancy.