Home pregnancy tests are about 99% accurate when used correctly, but several things can produce a result that doesn’t match reality. Some cause false positives (a positive result when you’re not pregnant), while others cause false negatives (a negative result when you are). Understanding the difference helps you figure out whether to trust your result or test again.
How Home Pregnancy Tests Work
Every pregnancy test detects a hormone called hCG, which the body produces after a fertilized egg implants in the uterus. Home tests vary in how sensitive they are to this hormone. The most sensitive option on the market, First Response Early Result, can detect hCG at concentrations as low as 6.3 mIU/mL, picking up over 95% of pregnancies by the day of a missed period. Other brands require concentrations of 25 mIU/mL or higher, meaning they catch fewer early pregnancies. Some widely sold tests need levels of 100 mIU/mL or more, detecting only about 16% of pregnancies at the time of a missed period.
This range in sensitivity is why two different tests taken on the same day can give you two different answers, and why the timing of when you test matters so much.
Causes of a False Positive
Chemical Pregnancy
This is the most common reason for a positive test that doesn’t lead to an ongoing pregnancy. A chemical pregnancy happens when a fertilized egg implants briefly and starts producing hCG, but stops developing very early, often before an ultrasound could detect anything. Roughly 25 to 30% of early positive pregnancy tests fall into this category. Because today’s tests are sensitive enough to detect very low hCG levels, they pick up pregnancies that would have gone unnoticed a generation ago.
After an early loss, hCG doesn’t disappear overnight. Levels drop by about 35 to 50% within two days and 66 to 87% within a week, but you can still test positive for a week to several weeks afterward. If you get a positive test followed by bleeding and a later negative test, a chemical pregnancy is the most likely explanation.
Fertility Medications
Some fertility treatments involve injecting hCG directly to trigger ovulation. Brand names include Pregnyl, Profasi, Novarel, and Ovidrel. If you take a pregnancy test while this medication is still in your system, the test will detect the injected hormone and show a positive result that has nothing to do with an actual pregnancy. If you’re undergoing fertility treatment, your clinic will tell you how long to wait before testing.
Evaporation Lines
Reading a test outside its designated time window is a surprisingly common source of confusion. Most tests instruct you to check results within two to five minutes. After that window, urine drying on the test strip can leave a faint, colorless line called an evaporation line. It sits where a positive line would appear, but it’s typically thinner and lacks the pink or blue color of a true result. If you come back to check a test 20 minutes later and see a faint mark, that’s almost certainly an evaporation line, not a positive.
Recent Miscarriage or Ectopic Pregnancy
Any pregnancy that recently ended, whether through miscarriage, ectopic pregnancy, or termination, leaves hCG in your system for a period of time. The same clearance timeline applies: levels drop significantly within the first week but can remain detectable for weeks. A positive test during this window reflects leftover hormone, not a new pregnancy.
Molar Pregnancy
A molar pregnancy is an abnormal growth of tissue in the uterus that produces extremely high levels of hCG, sometimes exceeding 100,000 mIU/mL. It develops from an abnormally fertilized egg and requires medical treatment. A molar pregnancy will produce a strong positive result on a home test, but ultrasound will show no normal fetus developing.
Certain Medical Conditions
In rare cases, the body produces hCG outside of pregnancy. The pituitary gland, a small structure at the base of the brain, can produce low levels of hCG, particularly in women who are perimenopausal or postmenopausal. This is a normal physiological process, but it can be enough to trigger a faint positive on a sensitive test.
Kidney failure can also elevate hCG levels because the kidneys are responsible for filtering it out of the blood. When they aren’t functioning properly, hCG builds up. Certain rare tumors, including some ovarian tumors and other germ cell tumors, also produce hCG as a byproduct. These are uncommon causes, but they’re worth knowing about if you’re getting unexplained positive results and you’re certain you’re not pregnant.
Causes of a False Negative
Testing Too Early
By far the most common reason for a false negative is testing before your body has produced enough hCG to reach the test’s detection threshold. After implantation, hCG levels roughly double every two to three days in early pregnancy. If you test several days before your expected period with a standard-sensitivity test, your levels may simply be too low. Waiting until the day of your missed period, or a few days after, dramatically improves accuracy.
Diluted Urine
Drinking large amounts of fluid before testing can dilute the hCG concentration in your urine below the test’s detection threshold. This is why most test instructions recommend using your first morning urine, which is typically the most concentrated.
The Hook Effect
This is a lesser-known cause of false negatives that happens later in pregnancy, not earlier. When hCG levels are extremely high (typically in the hundreds of thousands of mIU/mL), the sheer amount of hormone can overwhelm the test’s antibodies and paradoxically produce a negative or very faint result. Research has documented this in women whose pregnancies ranged from about 4 weeks to 16 weeks, with hCG levels as high as 268,000 IU/mL producing false negatives on standard point-of-care urine tests. The hook effect is rare and mainly relevant if you’re well into a pregnancy and get an unexpectedly negative urine result. A blood test resolves the confusion.
How to Get the Most Accurate Result
Use your first morning urine, when hCG is most concentrated. Follow the test’s time instructions exactly: check the result within the window specified (usually two to five minutes) and discard the test afterward. If you get a faint line within the correct time window, it’s most likely a true positive, since even a faint line means hCG was detected.
If your result doesn’t match what you expected, test again in two to three days. In a viable early pregnancy, hCG rises quickly enough that a follow-up test will typically show a darker line. If results remain ambiguous, a blood test can measure your exact hCG level and track whether it’s rising, falling, or holding steady, which tells a much clearer story than any home test can.