Pregnancy Test Calculator
Calculate pregnancy test reliability based on days after conception and hCG levels. Tests are most accurate 12-14 days after ovulation when hCG is elevated. Use this to understand test accuracy timing.
Formula
Accuracy = 5% (hCG<5) → 20% (hCG<10) → 50% (hCG<25) → 75% (hCG<50) → 95% (hCG<100) → 99% (hCG≥100)
- Conception is day 0; hCG appears ~6 days later (after implantation)
- hCG doubles every 48-72 hours in early pregnancy (slower after 8-10 weeks)
- Most home tests detect hCG at ≥25 mIU/mL
- Sensitive tests detect hCG at ≥10 mIU/mL
- Accuracy increases dramatically once hCG rises above detection threshold
- False negatives common if testing too early (hCG below threshold)
Example
Inputs
- Days After Conception (estimated): 14
- hCG Level (mIU/mL) - if known: 100
At 14 days after conception with hCG level of 100 mIU/mL, test accuracy is approximately 95-99%. This is well into the reliable testing window.
Frequently asked questions
When can I test for pregnancy?
Earliest reliable testing is 12-14 days after ovulation (typically first day of missed period). Testing earlier risks false negatives.
What is hCG?
hCG (Human Chorionic Gonadotropin) is produced by the placenta after implantation. It's what pregnancy tests detect.
Why do tests give false negatives?
Most commonly because hCG levels haven't reached the test's detection threshold yet. Wait a few days and retest.
How fast does hCG rise?
hCG typically doubles every 48-72 hours in early pregnancy, reaching easily detectable levels by 12-14 days after ovulation.
Can I test at 3 weeks?
At 3 weeks (21 days from LMP), hCG should be well above detection thresholds. Test accuracy should be very high.