How home pregnancy tests detect hCG
Home pregnancy tests detect human chorionic gonadotropin (hCG), a hormone produced by the developing trophoblast cells after the blastocyst implants into the uterine lining.
Following implantation (which occurs 6 to 12 days post-ovulation), serum and urinary hCG concentrations double approximately every 48 hours.
Test sensitivity dictates detection limits: Ultra-sensitive tests detect hCG at 10 mIU/mL (often 4 to 5 days before a missed period), standard drugstore tests detect 20 to 25 mIU/mL, and standard clinical office tests detect 50 mIU/mL.
Worked example: Test timing progression
A woman ovulates on October 1st. Implantation typically occurs on October 10th (9 DPO).
An early 10 mIU/mL test can detect hCG around October 13th (12 DPO) with ~90% accuracy, while testing on October 15th (14 DPO, day of expected period) achieves >99% accuracy.
Pregnancy test inputs
Known Date
Enter LMP, ovulation date, or expected period.
Test Sensitivity
Select early detection (10 mIU/mL) or standard (20–25 mIU/mL).
CALCULATOR QUESTIONS
Home pregnancy test timing & accuracy calculator questions
What causes a false negative pregnancy test?
The most common cause of a false negative is testing too early, before urinary hCG has accumulated to the test threshold. Other causes include diluted urine from high fluid intake and irregular ovulation.
Why is first-morning urine recommended?
Urine that has remained in the bladder overnight is the most concentrated, containing the highest density of hCG molecules for easy test strip binding.