How the Chemistry in Pregnancy Tests Works

If a trained technician uses a home pregnancy test, it has an accuracy rate of 97%. If an untrained woman uses one, the accuracy is much less. Why the difference? The technician is trained in precision and procedures, but, more important; she reads and follows the directions exactly. She knows how and why the test is supposed to work. She knows when it becomes effective. For best results, anyone who uses a pregnancy testing kit needs to understand how it works.

The test measures a hormone, human chorionic gonadotropin, HCG, which begins to be produced by the body soon after conception. The embryo produces a small amount, which increases quickly after it attaches to the placenta. Some of this hormone is excreted in urine, where it can be measured. In a normal pregnancy, the amounts of HCG in the system will double every two or three days at first. A qualitative test can show if a woman is pregnant or not. Another test, the quantitative test (usually given by a doctor) can show how the pregnancy is proceeding. If the quantity of HCG in the body is low, there may be danger of miscarriage or of an etopic (outside the uterus) pregnancy. If the reading is high, the woman may be carrying twins or more. The quantity of HCG should increase rapidly in early pregnancy, and then fall off slightly after the first three months.
The most common reason for a false negative is that the test is used before implantation takes place, or before enough of the pregnancy hormone has built up in the body to activate the test. The most sensitive tests can detect the hormone soon after implantation. The least sensitive tests won’t detect the hormone until six to twelve days after implantation, which can be as much as 21 days after ovulation. This is because after sexual relations, the sperm may remain in the fallopian tube for as much as five days before ovulation, and then the fertilized egg may take as many as sixteen days to attach to the placenta. Ovulation is usually, although not always, fourteen days after the last period. A false negative, therefore, is more unlikely after a missed period.

Diuretics can also decrease the percentage of HGC in the urine, producing a false negative, and so can promethazine, which is an antihistamine and sedative that is sometimes used to combat morning sickness. Just drinking too much water before the test may produce a false negative, although it’s unlikely.

A false positive result may occur because the test was used incorrectly. If the user waits too long to read the test, it may falsely indicate pregnancy. Another reason for a false positive may be that the woman has been given HCG by a doctor to promote fertility. The test may read this introduced HCG as induced by pregnancy. Stale test kits can sometimes give false positives. Certain drugs can also give false positives, such as anticonvulsants, anti-parkinsonian drugs, certain tranquillizers, and the same promethazine that can also lower readings.

Finally, in some cases, false positives can be a sign of illness. Certain cancers can prompt the body to secrete the hormone that makes the test read positive. IgA deficiency, an antibody deficiency that makes the body prone to infection, can also prompt a false positive, as can a variety of other uncommon disorders.

To get the clearest result from a pregnancy test, follow the directions exactly. Use the first urine of the day, because it’s the most concentrated, and time the test exactly. Wait until you have missed a period for the most certain answer, or be willing to retest after a week if you test early. Once you’ve got your result, make an appointment with your health care provider.