How Do Pregnancy Tests Work?

Pregnancy Test False Positives and Negatives

Positive Pregnancy Test
The pigment molecules in home pregnancy tests often are arranged so that a positive (pregnant) result forms a plus sign. Peter Cade/Getty Images

Pregnancy tests rely on the presence of the hormone human chorionic gonadotropin (hCG), a glycoprotein that is secreted by the placenta shortly after fertilization.

The placenta begins developing after the fertilized egg implants in a woman's uterus, which happens about six days after conception, so the earliest these tests can be used to detect pregnancy is about six days post-conception. Fertilization does not necessarily take place the same day as intercourse, so most women are advised to wait until they miss their period before trying a pregnancy test.

hCG levels double about every two days in a pregnant woman, so the test is much more reliable two weeks after conception than one week later.

The tests work by binding the hCG hormone, from either blood or urine, to an antibody and an indicator. The antibody will only bind to hCG; other hormones will not give a positive test result. The usual indicator is a pigment molecule, present in a line across a home pregnancy urine test. Highly sensitive tests could use a fluorescent or radioactive molecule attached to the antibody, but these methods are unnecessary for an over-the-counter diagnostic test. The tests available over-the-counter versus obtained those at the doctor's office are the same. The primary difference is the decreased chance of user error by a trained technician. Blood tests are pretty much equally sensitive at any time. Urine tests tend to be most sensitive using urine from early morning, which tends to be more concentrated (would have the highest levels of hCG).

False Positives and Negatives

Most medications, including birth control pills and antibiotics, do not affect the results of pregnancy tests. Alcohol and illegal drugs do not affect the test results. The only drugs that can cause a false positive are those containing the pregnancy hormone hCG in them (usually used for treating infertility).

Some tissues in a non-pregnant woman can produce hCG, but the levels are normally too low to be within the detectable range of the tests.

Also, about half of all conceptions don't proceed to pregnancy, so there may be chemical 'positives' for a pregnancy that won't progress. For some urine tests, evaporation may form a line that could be interpreted as a 'positive'. This is why tests have a time limit during which you should examine the results. It's untrue that urine from a man will give a positive test result.

Although the level of hCG rises over time for a pregnant woman, the quantity of hCG for one woman is different from the amount produced in another. This means some women may not have enough hCG in their urine or blood right at six days post-conception to see a positive test result. All tests on the market should be sensitive enough to give a highly accurate result (~97-99%) by the time a woman misses her period.