How Pregnancy Tests Work

The hormone is released into the bloodstream and some passes to the urine. A blood test is much more sensitive than a urine test. The blood pregnancy test can detect hCG as early as six days after the embryo has implanted in the uterus. There are no special preparations needed for the blood test. To collect a sample of blood for the pregnancy test the technician first wraps an elastic band around the upper arm to stop blood flow. This makes the vein enlarged, which makes it easier to get the needle in place. The site is cleansed with alcohol and the needle inserted. Blood is collected in a tube attached to the needle. After enough blood is removed, the needle is taken out and gauze and pressure is applied to the area to stop bleeding. There are rarely complications from a blood draw. Some people may develop a small bruise or phlebitis, which is an inflamed vein and is treated with warm compresses.

A urine test is much simpler and pain free, but is not as sensitive as the blood test. The best time of day to use a urine pregnancy test, widely available in discount and drug stores, is first thing in the morning. hCG will be at its highest level of concentration. To use a urine test later in the day, try to wait at least four hours after last urinating before using the test to get the best results. When taking a urine test at a lab or doctor's office, they will need around four tablespoons. Do not touch the container to your skin and keep toilet paper and other types of substances out of the container.

Today's home pregnancy tests use monoclonal antibodies to detect traces of human chorionic gonadotropin. These antibodies are coated with a bonding substance that attaches to the hCG hormone if it is present. The chemical formula then gives a positive or negative result, usually indicted by a color change or other indicators. It is important to follow the directions exactly as these tests are timed and if you wait longer than indicated, false results can occur.

There are factors to consider when taking a home pregnancy test or at the doctor's. If it is very early in the pregnancy or the test is done at a low time of day, hCG may not be detected. Diuretics can lower the hCG urine levels and be missed by the test. If a miscarriage occurred, hCG levels may remain high for up to four weeks afterwards. Other medications that prevent blood from clotting, hypnotics, antipsychotics or antinausea can interfere with test results so be sure to let your doctor know what you are taking.