How Pregnancy Tests Work

Home pregnancy tests work in the same basic way

Pregnancy tests detect the presence of a hormone called human chorionic gonadotropin, or hCG, which is produced by the developing placenta soon after an embryo attaches itself to the uterine lining. The amount of hCG present in a woman's body increases rapidly in the subsequent days. Home pregnancy tests are based on the presence of hCG solely in the urine, though your physician may also use a blood test. HCG is measured in thousandths of International Units, or mIU. The most sensitive tests can detect hCG levels as low as 15 to 25 mIU. This level is present in the urine of most pregnant women about eight to eleven days after ovulation or about the first day after a missed period. With less sensitive tests, you have to wait longer for an accurate result.

To make any test easier to interpret, experts say it's best to use first-morning urine, because it's the most concentrated. If this isn't possible, avoiding urination for several hours before testing can also be helpful. You should check for results when the instructions say to do so. For example, if you leave the test to sit for too long, an evaporation line may develop and look enough like a positive result to cause confusion.

Almost every brand of pregnancy test receives some negative consumer reviews at Epinions.com, because the test was negative on the first day of a missed period, then the woman tested positive a few days to a week later. Contributors who experienced this say that the test failed to work. However, medical experts point out that in as many as 10% of pregnant women, the embryo doesn't implant until after the first day of a missed period. Until implantation occurs and the placenta begins developing, there is no hCG in the body to be detected by a pregnancy test.

There are several other variables influencing the accuracy of a pregnancy test, including how well the instructions for use were followed. Another factor may be hCG already present in the body from a miscarriage, recent birth or some fertility drugs. Testing after drinking a large quantity of liquid may also result in a false negative, due to diluted urine. Detergent or soap residue can contaminate a collection cup. Exposing the test strip to heat, sunlight or vibration may affect results. Finally, excess protein or blood in the urine, as well as other medications like Thorazine, Dolophine or Methadose, can influence results.