While there are several different types of pregnancy tests available to diagnose pregnancy, all have one thing in common. All pregnancy tests look for the presence of a pregnancy hormone known as human chorionic gonadotropin, or hCG. This hormone develops as soon as a fertilized egg implants in the uterus, and grows exponentially during the first few months of pregnancy. Women who suspect they are pregnant should be aware of the many factors that can influence hCG reading and pregnancy test accuracy.
Test Timing and Home Pregnancy Tests
WomensHealth.gov writes that in 10 percent of women, implantation does not occur until much later--as late as the day after the day a period is due to begin. For this reason, the Mayo Clinic recommends waiting at least a week for more reliable results, despite the fact that many home pregnancy tests claim accuracy much earlier.
Blood Tests
There are two types of blood tests available to check for pregnancy. A quantitative blood test, known as a beta hCG, tests the exact amount of hCG in your blood, while a qualitative test simply looks for the presence of the hormone. The qualitative test is as accurate as the urine test, but the quantitative test is more sensitive, according to FertilityPlus.org, and can tell how far a pregnancy has advanced.
Maximizing Accuracy With a Home Pregnancy Test
Home pregnancy tests are highly reliable, according to the Mayo Clinic, when administered according to test instructions and when taken one week after a missed period. Women should use a test that hasn't expired, and wait a full 10 minutes before checking results. Testing urine first thing in the morning may increase test accuracy, as urine collected at other times of the day may have too much fluid and dilute the presence of hCG.
False Positives
Although rare, false positive results are possible. Traces of blood or protein in the urine, along with diuretics and promethazine used to treat nausea and allergies can create inaccurate results, according to the Mayo Clinic. An expired or damaged test kit may also produce a false positive result, as can certain fertility medications that contain the hCG hormone.
False Negatives
Taking the test too early before sufficient levels of hCG have developed can give a false negative result, according to the Mayo Clinic. So can waiting too long to use the urine collected for the test or waiting too long to read the results. Diluted urine, or urine that contains too much fluid, can weaken the detectable presence of hCG. This is why testing first thing in the morning when urine is most concentrated is best for increased accuracy.
