How the Chemistry in Pregnancy Tests Work

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"How the Chemistry in Pregnancy Tests Work"
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A missed period is one of the first signs of pregnancy, but by no means the only one. A woman who misses her period is doubtlessly anxious to find out if she is pregnant as soon as possible, even before making that crucial doctor's appointment, so she will likely turn to an home pregnancy test to confirm her suspicions.

If used correctly, many home pregnancy tests can be astonishingly accurate, over 90% accurate, in fact. At home pregnancy tests are sold for a mere ten dollars in drugstores worldwide, but ever wonder how a cheap little stick coated with urine can confirm that a woman's life is about to change, as she expects a new baby?

The chemistry and the technology of the test is actually quite simple. As soon as the woman's egg is fertilized and implanted in her uterus, usually but not always 6-12 days after ovulation, her body begins secreting the, hormone human chorionic gonadotrophin, (hCG) in her urine. This hormone is used to form the fetus' placenta and excessive quantities are only present in pregnant women.

Her hcg levels build up and are detectable in her blood and urine, and hcg levels double every two days. Maximum hcg levels are reached at about the eighth week of pregnancy. While this hormone is also manufactured normally in the human body, the hcg levels in non-pregnant women are so minute they are not even detected in at home-pregnancy tests.

By the time the woman's period is due to arrive, there are hcg levels in the urine that are high enough to be detected in an at home pregnancy test. The pregnancy test measures hcg levels in the urine, through antibodies placed in the test's stripes. Within minutes, the antibodies, man-made molecules that bond specifically and uniquely to the hcg hormone, will react if hcg is present in quantities that are pre-determined by the test. The hcg will soak the stick and react with the present antibodies, causing a color change or line to appear on the stick. Depending on the level of hcg detected, the color change may vary in intensity.

Not all pregnancy tests are created alike, as some are more sensitive than others. Medically speaking, hcg is measured in thousandths of international units, or miu, which is a standard unit of measurement. Some tests are manufactured with sensitive antibodies that react to ultra low hcg levels, 20 to 25 miu of hcg in the urine. These tests can be accurately used up to three days before the woman's period is due.

Other tests are less sensitive, and require a minimum of 50 to 100 miu and can only be taken after the woman's period has been delayed for more than three days.
But despite their accuracy, pregnancy tests are not fool-proof. Although at home pregnancy tests do not usually produce false positives, false negative results are common. This is often due to:

*Testing Too Early. Pregnancy is not a precise science. Implantation of a fertilized egg can take place up to 10 days after ovulation, meaning hcg quantities in the urine are too low to be to be detected if an at home pregnancy test is taken too early. Even when taking early, ultra sensitive pregnancy tests, it is it is better to test after monthly menstruation is missed, to ensure the test's accuracy.

*Using the Pregnancy Test While Taking Medications that Contains Hcg. While birth control and antibiotics will not affect the test's outcome, some medication that contains the hcg hormone can produce a false positive. Consult your physician to verify that your drug therapy does not interfere with the pregnancy test's result.

*Using Insufficient Urine or Taking the Test improperly. If the urine is not properly submerged under the stick for the specified period of time, or minute quantities of urine are used, the test will not be able to detect the hcg, even if significant amounts are present.

*Drinking Too Much Water Prior to Testing. Water dilutes the hcg hormone, making it immeasurable. Using first morning urine, when the hcg hormone is at its peak level, is best when taking an at home pregnancy test.

*Reading the Result too Early or Too Late. The result must always be read within the specified time frame mentioned in the test, usually within three to seven minutes of taking it.

Regardless of the outcome, see your doctor and have him or her perform a clinical blood test. Following up all at home pregnancy results with a blood test performed by a physician is the surest confirmation of pregnancy.

More about this author: Paola Fanutti

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