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False Positive Pregnancy Test

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You’ve read that home pregnancy tests (HPT) are nearly 100% accurate, so how can it be that your period started less than a week after you finally got a big fat positive (BFP) on a pregnancy test? Discouraged and sad, you are probably wondering if the test was wrong, giving you a false positive result and filling you with false hope.

It is true that most brands of home pregnancy are 97-99% accurate. So (not that this will necessarily make you feel better), it is not likely that the test was “wrong”, at least in technical terms, in showing a positive result for pregnancy. This assumes, of course, that the test was performed according to the directions. It is important to keep in mind that home pregnancy tests work by detecting a specific level of hCG (human chorionic gonadotropin), the “pregnancy hormone”, in your urine. The placenta will begin to secrete hCG immediately after the embryo implants into the uterine wall, and the amount of hCG will increase exponentially in the first weeks of pregnancy. While non-pregnant women do have a very small amount of hCG (under 5 mIU/ml) circulating in their blood, home pregnancy tests are not sensitive enough to detect this low level of hCG. Therefore, a positive result will appear if and only if the test detects enough hCG to indicate pregnancy. (By the way, this is also true for blood pregnancy tests performed at a lab.) But, the question still remains: Why did I receive a positive result, and then start my period a few days later? Here are a few possible explanations for a seemingly false pregnancy test:

Chemical pregnancy: Probably the most likely explanation is that you did in fact conceive successfully, but, unfortunately, suffered a very early miscarriage, often referred to as a chemical pregnancy. Fertility experts suggest that up to 70% of conceptions end in miscarriage. Many women experience a chemical pregnancy without ever realizing it, as the miscarriage occurs soon after conception and the menstrual period arrives on time. However, for the trying-to-conceive crowd, it is not likely that a pregnancy, of any type, will go unnoticed. With early detection pregnancy tests, it is possible to confirm pregnancy just days after ovulation. Clearly, the excitement that comes with confirming pregnancy with an early detection home pregnancy test must be balanced with some awareness of the reality that many pregnancies end very early on.

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hCG trigger shots: Fertility specialists commonly give shots of hCG to stimulate ovulation. Used for this purpose, hCG acts like luteinizing hormone and helps to rupture the follicle carrying the egg to nudge ovulation along. Depending on the dosage of the hCG trigger shot given, it can take up to two weeks for the hCG from the shot to clear the body. As a result, receiving an hCG shot in the course of your cycle could potentially produce a false pregnancy test. To ensure accuracy of your HPT following an hCG trigger shot, keep in mind the following guidelines: wait at least 14 days after a 10,000 IU injection, 10 days after a 5,000 IU injection, or 7 days after a 2,500 IU injection.

Evaporation lines: The instructions that come with your HPT will request that you read the results of the test within a given time frame (typically 5 minutes). The reason for this is so that you do not confuse the appearance of urine evaporation lines with a faintly positive test line. It is not at all unusual for evaporation lines to appear on the test strip as your urine dries. If you read the test outside of the recommended time frame, it is possible that the “positive” result you see is simply an evaporation line.

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