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How Do Pregnancy Tests Work?

Basic Information

If your egg was fertilized by your partners sperm, the fertilized egg will travel into the uterus and implant in the uterine lining. When this occurs, the fertilized egg then starts secreting the pregnancy hormone called human chorionic gonadotrophin (HCG).

HCG can be found in your urine as early as 6 days past ovulation. Very sensitive home pregnancy tests can detect your pregnancy at about 9 or 10 days past ovulation in a significant percentage of cases.

How the Tests Actually Work

The pregnancy tests on the market today are manufactured with monoclonal antibodies that detect minute traces of HCG.

When you take a home pregnancy test, you need to soak a portion of the test in your urine. As your urine moves up the test into the testing area, you will see a control line which is always present and, if you are pregnant, another line. This line is caused when the HCG in your urine reacts with the monoclonal antibodies, creating a distinct color change. The color of this line will vary in intensity based on how much HCG is in your urine.

Tips For Getting Accurate Results

Be sure to read the test instructions carefully, taking special notice of the test reading time. Most home pregnancy tests will tell you not to read your test after a certain amount of time. If a line appears after this time period, it should not be counted as a positive pregnancy test. What you may see is an evaporation line, usually colorless, but slightly darker than the color of the pregnancy test membrane. Evaporation lines can occur as your urine dries on the test.

Using first morning urine is a good way to make sure the HCG in your urine is concentrated, making it easier for the pregnancy test to return a positive result if you are pregnant.

If you want to test in the middle of the day, you will want to limit fluid intake for several hours to make sure your urine is not diluted.


Last modified: February 10, 2013


The information provided here should not be considered medical advice. It is based on the average experience of women trying to conceive and may not be what you may be experiencing. It's not meant to be a replacement for any advice you may receive from your doctor. If you have any concerns about your cycle or our ability to get pregnant, we advise you to contact your doctor.