A false positive pregnancy test occurs when a home test indicates the presence of the pregnancy hormone, Human Chorionic Gonadotropin (HCG), even though a viable uterine pregnancy does not exist. Home pregnancy tests work by detecting HCG, a hormone normally produced by the placenta shortly after a fertilized egg implants. While most modern tests are highly accurate, receiving two consecutive false positives is medically possible, though rare. When this happens, the cause is typically a specific, persistent factor causing HCG detection, not random error.
Understanding Why a Single False Positive Happens
Most instances of a single false positive result are due to issues with the test kit or user error, rather than an underlying medical condition. A frequent cause is reading the test after the designated time limit has passed. The test instructions specify a precise window for reading the result, typically around ten minutes.
If the test is left to sit for too long, a faint, colorless line known as an evaporation line may appear as the urine dries. This evaporation line can be mistaken for a true positive result, which is indicated by a line that has color. Some tests are overly sensitive or defective, meaning they may react to trace amounts of non-HCG antibodies or other substances in the urine. These factors cause a temporary issue usually resolved by retesting with a different brand or a fresh sample.
Medical Conditions That Elevate HCG
The most concerning causes of multiple false positives involve the body producing HCG or a substance that mimics it, even in the absence of a continuing pregnancy.
One common reason is a very early pregnancy loss, often called a chemical pregnancy. Implantation occurs just long enough for the embryo to begin producing HCG, resulting in a positive test. The pregnancy ends shortly after, often before a missed period, and the positive result lasts only until hormone levels fall back to zero.
Another situation that generates HCG but is not a viable pregnancy is an ectopic pregnancy, where the fertilized egg implants outside the uterus, most often in a fallopian tube. This non-viable and dangerous pregnancy still produces HCG detectable on a home test. Similarly, a molar pregnancy, or gestational trophoblastic disease (GTD), involves the abnormal growth of cells that produce very high levels of HCG. These are positive tests that do not correspond to a healthy uterine pregnancy.
Certain medications can also interfere with test accuracy, most notably fertility treatments containing HCG, commonly known as “trigger shots.” These injections are administered to trigger ovulation, and the HCG from the shot can remain in the bloodstream and urine for up to two weeks, causing a false positive reading.
Beyond this, very rare medical conditions can cause the body to produce HCG, including certain tumors of the pituitary gland, ovaries, or other organs. Some non-gynecologic cancers, such as those of the liver, stomach, or lung, can also cause an elevation of HCG that results in a positive test.
When to Confirm Results with a Doctor
When multiple positive home pregnancy tests are received, it is important to confirm the result with a healthcare provider, especially if there is no corresponding viable pregnancy.
The most definitive step is a quantitative HCG blood test, which measures the exact level of the hormone in the bloodstream, often reported in milli-international units per milliliter (mIU/mL). A level below 5 mIU/mL is considered negative for pregnancy, while a level above 25 mIU/mL is considered positive.
If the blood test confirms a low but positive HCG level, the doctor will repeat the test after 48 to 72 hours to see if the levels are rising or falling. In a healthy early pregnancy, HCG levels double about every three days.
If the levels are falling, it suggests a recent pregnancy loss, like a chemical pregnancy, or residual hormone from a recent event. If the HCG levels are elevated and pregnancy is ruled out, a medical investigation is necessary to identify the underlying cause, such as medication interference or a rare HCG-producing condition.