The accuracy of a pregnancy test after a miscarriage can depend on a variety of factors. Women who were further along in their pregnancies will have higher hormone levels in their bodies than those who miscarried earlier, and those levels will take longer to drop. A test taken shortly after the miscarriage will be more likely to show a false positive than one taken later. Over-the-counter tests, which show positive or negative only, may be less accurate than a repeat blood test that shows actual hormone levels and how they are changing. In general, a pregnancy test after a miscarriage has a good chance of showing a false positive result.
Pregnancy tests show results based on the amount of human chorionic gonadotropin, or hCG, in the blood or urine. Levels of this hormone increase rapidly during early pregnancy, so the further along the pregnancy is, the higher the level will be. Therefore, women whose pregnancies have progressed further will tend to have more hCG, and it will take more time for their level to drop back to zero. Depending on the quantity of hCG present, it can take anywhere from a week to almost a month for a pregnancy test after a miscarriage to be accurate.
Timing of a pregnancy test after a miscarriage is also critical for a valid result. Due to the variability in hCG levels women may have as well as individual differences in the rate they may drop, it can be hard to determine how soon after the pregnancy is lost that a test may come up negative. A test taken within the first week after a pregnancy loss will likely give a positive result since hCG levels will have had little time to drop; for some women, this may continue to be true for several weeks.
Different types of pregnancy tests may offer more accuracy than others when used after a miscarriage. Over the counter tests come with varying degrees of sensitivity to hCG levels in the urine, so one test may still read positive as hormone levels drop while others will stop registering them. For truly accurate results, the best option is often to have a series of blood tests so the results can be compared. These tests show quantitatively how much hCG is in the blood, so if the numbers decrease over time, it is typically a clear indication that a miscarriage has occurred.