There are also a few medications that can interfere with the result. Rather than triggering a false negative, they can sometimes return a false positive reading, leading to you believe that you are pregnant when, in fact, you are not.
HCG Medications
Some of the drugs used during in vitro fertilization (IVF) or intrauterine insemination (IUI) can interfere with hCG measurements because they are forms of hCG. Women unable to ovulate on their own are often prescribed a fertility medication such as Clomid (clomiphene citrate). After completing the five-day course, a “trigger shot” of Pregnyl, Ovidrel, or Novarel may be used to mature eggs and induce ovulation. Typically, it takes about 10 days for the synthetic hCG in these trigger shots to clear from your blood and urine. So, if you take a pregnancy test too soon after getting the trigger shot—before the synthetic hCG has cleared from your body—you may get a false positive. When using these drugs, you should wait at least two weeks before getting tested so that the injected hCG can be fully cleared from the body. Most doctors, however, advise against home testing and ask that you come to the office to get a blood-based test. While the blood test also detects hCG, it is far more accurate than a home urine-based test.
HMG Medications
Another type of injectable drug, known as human menopausal gonadotropin (hMG), can be used to aid in ovulation. HMG is made up of two hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Common hMG medications include Menopur, Pergonal, and Repronex. When used in the IVF or IUI setting, the LH component can also trigger a false positive result but for different reasons. While an hCG injection triggers a false positive by introducing synthetic hCG into the system, a surge in LH can do the same because the two hormones are so structurally similar.
False Negative Pregnancy Tests
If you’ve taken a pregnancy test and believe that a negative result is wrong, the best thing to do is wait for at least two days and test again. Often, if you test too early, the hCG levels will be below the threshold of detection of many home pregnancy tests. HCG levels are typically expected to double every two days if you are pregnant. If you are pregnant and you wait another two days before testing again, the levels should double again, increasing your chance of an accurate reading. If, on the other hand, you are undergoing IVF or IUI, it is best to follow your doctor’s instructions and wait to get an in-office blood hCG test. While having to hold off two weeks can add anxiety to an already emotional process, the risk of a false positive test may be more upsetting than waiting for an accurate reading.
