So you just tested for pregnancy, but don’t quite understand or believe the results? In this article, OBGYN Dr. Mare explains false positives, false negatives, how often they occur, what they mean, and next steps.
If you’ve ever taken an at-home pregnancy test, then you know that from what’s on the boxes, the results should be cut and dry: either you're pregnant or you're not pregnant. But as always, it's not quite that simple. While the majority of home pregnancy tests do a good job of recognizing a pregnancy, there's always the possibility for false results.
OTC (over-the-counter) pregnancy tests work by detecting human chorionic gonadotropin (hCG) in the urine. This is the pregnancy hormone, which can be measured in both urine and blood. The urine test only detects the presence of hCG while the serum test detects the actual amount of hCG.
Instructions on taking an OTC pregnancy test vary as do the way results are presented. Some tests use a plus vs. minus sign, one vs. two indicator lines, or the words “pregnant” vs. “not pregnant.” Of note, for all pregnancy tests, even a faint plus sign or line is considered a positive result.
For all pregnancy tests, even a faint plus sign or line is considered a positive result.
As if taking a pregnancy test isn’t already nerve wracking enough when you’re trying to conceive, it becomes more so if you aren’t sure you can trust the results. These false results can either be falsely negative (i.e., the test reads as negative, but you are actually pregnant) or falsely positive (i.e., the test reads as positive, but you are not pregnant).
It’s difficult to give a specific number in terms of how many home pregnancy test results are wrong simply because these aren’t reported, so we can’t study it. However, based on patients seen in emergency rooms, false negative results are more common, though they still occur at a rate less than 5%. False positive results appear to be even rarer.
Can a pregnancy test be wrong?
Pregnancy test false negatives
There can be several reasons behind this, so let’s go through them. False negative test results can be due to the following:
- Timing: this is probably the most common cause of a negative test. A lot of OTC pregnancy tests claim they can detect pregnancy before a missed period, but most don’t actually live up to that claim. This study of six OTC tests found that several of them had a lower threshold than claimed.
- Sensitivity: to some degree, this falls under timing as well. Every brand of pregnancy test has a numeric value of hCG at which it is supposed to detect it. Once your hCG level is at or above that level, you will get a positive result. Since this number varies between pregnancy test brands, one brand’s test may be negative while another will not. See our chart below for the best compilation of pregnancy test brands and their thresholds.
- Insufficient/diluted urine: this can happen when someone simply doesn’t urinate enough or when they overhydrate (the easiest way to tell this is if your pee is so clear that you can’t tell there’s any in the toilet bowl). In both cases, there is not enough of a concentrated urine sample for the test to detect the level of hCG in the urine. Since this is just an issue with the sample, it’s reasonable to repeat the test again shortly after once you can provide an adequate sample.
- Not giving the test enough time: this one’s pretty simple. Just make sure to follow the instructions on how much time the test needs in order to work.
If you have a negative pregnancy test, but think you might be pregnant, repeat the test one week after your missed period or talk to your health care provider about getting a blood test. This test is the most definitive chemical test for pregnancy.
Pregnancy test false positives
What about false positives? There can be a few causes for this too.
- Evaporation line: most pregnancy tests have specific instructions on how long to wait before reading the results. They also have a time limit after which the test can no longer be read (anywhere from 10 to 30 minutes). If a test is read after that recommended time limit, it may result in what looks like a positive test result; however, in most cases, this is not a positive pregnancy test, but an evaporation line left by the urine after that length of time has passed.
- Previous pregnancy loss: after a miscarriage or abortion, hCG starts to fall slowly over a period of 4 to 6 weeks, ultimately reaching a non-pregnant level (<5). The exact timing of this varies depending on how far along the pregnancy was at the time of the loss and how the loss occurred (procedure vs spontaneous). Taking a pregnancy test within this time period can lead to a false positive result.
- Chemical pregnancy: technically, this is not a false positive. In this case, the positive pregnancy test indicates a very early miscarriage that may not have progressed far enough to cause more than spotting or light bleeding. The only way to truly confirm this is with a serum hCG test.
- Medications: most medications don’t affect pregnancy test results. The ones that do typically contain hCG as an active ingredient (mostly infertility drugs) and therefore can interfere with pregnancy tests. A serum hCG test is the better option in these cases.
- Molar pregnancy: this is a serious, but rare one. A molar pregnancy (hydatidiform mole) is an abnormal pregnancy caused by genetic abnormalities at conception. There are two type of moles: complete and partial. Both are abnormal and require urgent removal with a procedure called a dilation and curettage. Afterwards, hCG levels are monitored closely to ensure they return to normal.
- Certain medical conditions: these are also very rare and include disorders affecting the pituitary gland, certain cancers including gestational trophoblastic disease (a rare sequela of molar pregnancy), kidney disease, and phantom hCG (where a person has antibodies that bind the urine test).
If you have a positive pregnancy and you’re sure you’re not pregnant, you can repeat it, but a persistently-positive pregnancy test, especially when accompanied by symptoms such as abdominal pain and heavy vaginal bleeding, could mean something more serious and should prompt a visit to your healthcare provider.
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