Pregnancy: How Do Pregnancy Tests Work?

I think that I might be pregnant. Please tell me how pregnancy tests work.

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Peg Plumbo CNM

Peg Plumbo has been a certified nurse-midwife (CNM) since 1976. She has assisted at over 1,000 births and currently teaches in the... Read more

There are several varieties of pregnancy tests -- some are done using urine and some, blood. Pregnancy tests can be performed at home or can be done in a lab. All of the commercially available tests detect human chorionic gonadotropin or hCG, a "glycoprotein." This substance, produced by placental tissue, can be detected once the fertilized ovum is implanted in the uterine wall. A sensitive blood or urine test detects hCG as early as eight to nine days after ovulation. The plasma concentration of hCG doubles at least every two days, peaks between days 60 and 70 of pregnancy and then declines.

Home Pregnancy Test

When using a home pregnancy test you will either need to urinate in a cup and then place a small amount of urine, using a dropper, into the testing well, or urinate directly on a testing stick. If the urine contains hCG, it will react with the antibody and a colored line will form. If performed according to instructions, home pregnancy tests claim 97 percent accuracy, but one study showed a 25 percent rate of false negatives and a significant (though smaller) number of false positives. These were probably due to poor technique, performing the test too close to conception or use of an outdated test.

Quantitative hCG Test

Sometimes called the Beta hCG blood test, this is done in a lab, measures the amount of hCG in your blood, detecting even very small levels.

Qualitative hCG Test

This blood test, also performed in a lab, measures the presence or absence of hCG in your blood, giving you a simple yes or no answer.

Whether testing urine or blood, there can be a variance in what is considered to be a positive pregnancy test. Though levels for positive can be set at 5, 10 or 25 units, a level under 5 is always considered negative.

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