Pregnancy Test

Overview

Pregnancy tests are used to determine whether or not a woman is pregnant. The two most common ways to test for pregnancy in the earliest stages are urine and blood tests. A urine test can be performed in a doctor’s office or at home, and a blood test – often as a follow-up to a positive result on a home test – is performed in a doctor’s office. Both of these tests work by detecting the antigen human chorionic gonadotropin, also called hCG, in the urine and blood stream. The hCG is produced in the body after the egg implants – about 10 to 12 days after fertilization occurs – and quickly increases in concentration in both the blood and urine. While home pregnancy tests are inexpensive and generally considered reliable, there are sometimes problems with the results. It is recommended that a positive result be followed up by a visit to a physician to confirm test results. In addition, continued symptoms after a negative result should be followed up with another test a week later or a visit to a physician.

Main Benefits

There are several benefits to home pregnancy tests. These tests are far less expensive than a doctor’s visit and are a wonderful option for women who are trying to conceive and wish to test frequently. This can be especially useful in cases where women have irregular periods and are trying to conceive. Tests are easy to use and require no expertise. Instructions come with the testing kits and, if followed correctly, tests are considered reliable. Privacy is also a consideration with home pregnancy tests. Finally, home pregnancy tests can often detect even trace amounts of hCG, making these tests one of the earliest indicators of pregnancy for most women after a missed period. A blood test is often done as a follow-up to a positive result, or if symptoms continue after a negative result, for confirmation.

How it works

After a fertilized egg implants in the uterus, hCG production begins. Pregnancy tests work by detecting the presence of this antigen in the blood or urine. In home pregnancy tests, an indicator that binds to hCG is present in the test. The test strip is held in the urine stream or in a container with collected urine. If the indicator reacts, the test confirms the presence of hCG. This is seen on the test as a positive result, and it is often seen in the form of a pink line or a plus sign. A blood test checks for the presence of the same antigen and is considered even more reliable than urine tests much of the time.

Mistakes, False Negatives and False Positives

While pregnancy tests are considered 99 percent reliable if used correctly, common mistakes can reduce the accuracy. Certain circumstances can also cause incorrect results including false negative results and, less commonly, false positives. For instance, waiting too long, or not waiting long enough, to check the results can invalidate the test. In addition, using diluted urine – or not holding the test in the urine stream long enough – can cause incorrect results. It is important to read carefully and follow all directions included for an accurate test. False negative results are a common problem when testing is done too early. Human chorionic gonadotropin is not present in the blood or urine before the fertilized egg has implanted. This implantation usually occurs around the same time a woman would expect a period. Thus, the most accurate results can be experienced one week after a period is due. Finally, false positives are very rare, but they do occasionally happen. These inaccurate results may be caused by the use of fertility drugs to encourage pregnancy, or by residual hCG in the blood or urine after a recent miscarriage or birth.

More information

Mayo Clinic: http://www.mayoclinic.com/health/home-pregnancy-tests/PR00100

Wikipedia: http://en.wikipedia.org/wiki/Pregnancy_test

Medline Plus: http://vsearch.nlm.nih.gov/vivisimo/cgi-bin/query-meta?v%3Aproject=medlineplus&query=pregnancy+test&x=0&y=0