Although rare, pregnancy tests can give false-positives, according to Mayo Clinic. This could be the result of the loss of the pregnancy shortly after the egg implanted in the uterine wall or as a result of taking the test too soon after taking fertility drugs that contain HCG.
A much more likely outcome to pregnancy tests is a false-negative, states Mayo Clinic. The earlier a woman takes a pregnancy test after a missed period, the greater the chance of the test showing a false-negative. Checking the test too soon or using diluted urine can also return a false-negative. To ensure the pregnancy test is as accurate as possible, a woman should follow the instructions closely and wait at least a week after a missed period to take the test, according to WebMD.