Pregnancy testing
False Negative Pregnancy Tests
A false negative occurs when a pregnancy test returns “not pregnant” even though conception has occurred. Understand why it happens, what to do next, and when to contact your healthcare provider.
How to Use This Guide
Use these steps to troubleshoot a negative result when pregnancy is still suspected:
- Check the timing of your test relative to ovulation and missed period.
- Review sample collection instructions (first-morning urine, storage, expiration).
- Consider repeating the test or requesting a blood hCG test from your provider.
Formula
hCG concentration (urine) = Serum hCG × Urine concentration factor × Dilution factor
False negatives occur if urine hCG < detection limit (typically 15–25 mIU/mL) or if the hook effect (very high hCG) blocks antibody binding.
Serial testing every 48 hours should show doubling hCG in early viable pregnancy.
Full Description
Most home pregnancy tests detect human chorionic gonadotropin (hCG) once it reaches 15–25 mIU/mL in urine. Testing too early, diluted urine, user error, or a faulty test can all yield false negatives. Rarely, high hCG concentrations cause a “hook effect,” outcompeting antibodies and preventing signal development.
Implantation typically occurs 6–10 days after ovulation, and hCG doubles every 48 hours thereafter. If a test is negative but your period is late, repeat testing in 2–3 days or request a quantitative serum hCG assay. Seek urgent care if you experience severe abdominal pain, shoulder pain, dizziness, or heavy bleeding—these can signal ectopic pregnancy or miscarriage.
Frequently Asked Questions
How soon after a missed period should I retest?
Wait 48–72 hours. hCG levels rise quickly; a repeat test often becomes positive if you are pregnant.
Can medications cause a false negative?
Most medications do not interfere. Diuretics and antihistamines can dilute urine. Fertility medications with hCG affect results only if taken recently.
Should I test first thing in the morning?
Yes. First-morning urine is more concentrated and increases the likelihood of detecting hCG.
When should I see a doctor?
If you have pregnancy symptoms with repeated negative tests, irregular bleeding, or pain, schedule a medical evaluation promptly.