Pregnancy tests work by detecting the presence of the pregnancy hormone human chorionic gonadotropin (hCG) in a woman’s urine or blood. This hormone is produced by the placenta shortly after implantation occurs, which is about 6-12 days after fertilization. Pregnancy tests marketed for home use are designed to be very sensitive, meaning they can detect even low levels of hCG. This allows them to confirm a pregnancy very early on.
What is a false positive pregnancy test?
A false positive pregnancy test occurs when the test detects hCG, but the person is not actually pregnant. There are a few different reasons why this can happen:
- Chemical pregnancy – Early miscarriage before a pregnancy can be confirmed.
- Ectopic pregnancy – When the fertilized egg implants outside the uterus.
- Ovarian tumors – Certain non-cancerous ovarian tumors may secrete hCG.
- Perimenopause – Hormone fluctuations in the years leading up to menopause.
- Medications – Some medications can cause elevated hCG levels.
- Evaporation lines – Lines that occur as the urine sample dries, not true positives.
- Test errors – Faulty test strips can occasionally give false positives.
The most common causes of false positive pregnancy tests are chemical pregnancies, evaporation lines, and test errors.
How accurate are home pregnancy tests?
Modern home pregnancy tests are very accurate when used properly. However, no test is 100% perfect. Here’s how accuracy stacks up:
- Accuracy when used correctly is 97-99% in most cases.
- False negatives are more common than false positives.
- Results are less accurate when testing very early or with diluted urine.
- Reading the result after the recommended time can cause errors.
- No brand is 100% perfect, but errors are uncommon from major brands.
To get the most accurate result, take the test first thing in the morning using concentrated urine. Wait the directed amount of time before reading it, typically 3-5 minutes. Retest if you get an unexpected result.
What percentage of positive pregnancy tests are false?
Various studies have tried to determine the overall rate of false positive pregnancy test results. The estimates range widely, but the average rate is fairly low:
- One study found a false positive rate of 0.8-3.6% with first morning urine.
- Another study estimated false positives occur in 0.1-0.2% of tests overall.
- Research of 117 women found 5.1% had false positives initially.
- Estimates range from as low as 0.1% up to 8% in some circumstances.
In the vast majority of cases, a positive pregnancy test means a pregnancy is present. But false positives are possible, especially with certain medical conditions or improper test use.
Factors that increase chances of false positives
While false positives are uncommon overall, there are some factors that seem to increase the chances of getting an inaccurate positive test result:
- Perimenopause – Hormone changes may increase hCG.
- Taking hCG injections – Fertility treatments raise hCG levels.
- Pituitary disorders – Can increase production of hCG.
- Ovarian cysts or tumors – May secrete hCG in rare cases.
- Reading the test after the recommended time.
- Evaporation lines – Urine drying on the test area.
- Diluted urine samples – Light or watery urine.
- Defective test strips – Manufacturing errors.
If you get a positive result but have risk factors for a false positive, retest with a different kit to confirm.
What percentage of negative pregnancy tests are false?
False negative pregnancy tests are more common than false positives. Here are the general estimates:
- False negatives occur in 10-25% of all tests, based on various studies.
- One research review estimated false negatives in 10% of tests.
- At-home urine tests have a false negative rate of 9.4-24.3%.
- False negatives are more likely with early testing before missed period.
Common causes of false negative pregnancy test results include:
- Too early testing – Not enough hCG hormone yet.
- Incorrect test use – Reading it too late, etc.
- Diluted urine – Drinking extra fluids before testing.
- Medical conditions – Ectopic pregnancy, ovarian cysts.
- Some fertility treatments – Can affect test accuracy.
To avoid false negatives, retest 1-2 weeks after a missed period for an accurate result. First morning urine will be most concentrated.
Can you be pregnant with a negative test?
It is possible, though uncommon, to still be pregnant even if a home pregnancy test shows a negative result. Here’s some key information about pregnancy with negative test results:
- Early testing often causes false negatives due to low hCG levels.
- Average hCG levels at time of missed period are 25-80 mIU/mL.
- Many tests cannot detect hCG under 25 mIU/mL.
- It may take up to 12-14 days after conception for hCG to rise enough.
- Ectopic pregnancy can also cause negatives if hCG is too low.
Retesting 1-2 weeks after a missed period will almost always provide an accurate result. At that point, a negative test result is correct more than 99% of the time.
Likelihood of negative but still pregnant:
- 4-5 days before period: Up to 25% chance of false negative.
- Day of missed period: 10-15% chance of false negative.
- 1 week after: Less than 5% chance.
- 2+ weeks after: Less than 1% chance.
In rare cases, ectopic pregnancy can still cause a negative test. Women with unusual abdominal pain and bleeding should see a doctor even with negative results.
At what point can pregnancy tests be trusted?
Home pregnancy tests offer accurate results as early as 4-5 days before a missed period. However, false negatives are fairly likely at that point. Here’s when pregnancy tests become very reliable:
- Day of expected period: ~85% accurate.
- 1 week after: ~95% accurate.
- 2 weeks after: ~99% accurate.
- 3 weeks after: ~99.9% accurate.
Testing too early with diluted urine is the number one cause of false negatives. For the most reliable results, test first thing in the morning 1-2 weeks after a missed period.
What hCG level is detectable?
Urine pregnancy tests measure hCG in International MilliUnits per milliliter (mIU/mL). Most tests can detect hCG between 10-25 mIU/mL. Levels during early pregnancy:
| Days Past Ovulation | Average hCG Level (mIU/mL) |
|---|---|
| 7 | 5 |
| 10 | 25 |
| 14 | 100 |
| 21 | 300 |
| 28 | 1000 |
Waiting until hCG passes 25 mIU/mL results in a much more reliable test. This typically occurs 10-14 days after conception.
How to avoid false positives and false negatives
While no home pregnancy test is perfect, you can take steps to lower the chances of a faulty result:
- Read and follow all test directions carefully.
- Use concentrated morning urine and avoid excess fluids before testing.
- Wait to test until at least the day of a missed period, ideally longer.
- Don’t read results after more than 5 minutes; evap lines can look positive.
- Retest with a different kit if the result is unclear or unexpected.
- Consider seeing a doctor for a blood test for ongoing uncertainty.
Testing too early is the #1 cause of inaccurate results. First morning urine will have the highest hCG levels as well.
Tips for early pregnancy testing
It’s best to wait until at least 10-14 days after ovulation to test. However, if testing early here are some tips:
- Use a sensitive test designed for early detection.
- Test with concentrated morning urine.
- Test every 48 hours to check for increasing hCG levels.
- Consider using a digital test to avoid deciphering faint lines.
- Confirm results with a quantitative blood test if still unsure.
With these precautions, early home pregnancy testing can be fairly accurate. Always retest until after a missed period though for maximum reliability.
Conclusion
False positives and negatives on home pregnancy tests are uncommon but can occur. The vast majority are accurate when used properly. To maximize reliability:
- Wait to test until 1-2 weeks after a missed period.
- Use tests with sensitive 10-25 mIU/mL hCG detection.
- Read directions carefully and don’t check results late.
- Retest if the result is unexpected or unclear.
Taking a pregnancy test can be an emotional experience. While false results are rare, a doctor can perform a quantitative blood test to provide definitive confirmation either way.