Understanding a Blood (Quantitative hCG) Pregnancy Test
A blood pregnancy test measures quantitative beta human chorionic gonadotropin (hCG), the hormone produced by the placenta shortly after implantation, and reports it as a concentration in mIU/mL. Unlike a urine test's yes/no result, a quantitative number can be classified against standard cutoffs and — when drawn twice, roughly 48 hours apart — used to calculate how quickly hCG is rising or falling, which is one of the ways clinicians gauge early pregnancy viability and location.
How the doubling-time calculation works
Enter the first hCG reading, the second hCG reading, and the number of hours between the two draws. The calculator finds the percent change between the two values, then treats the change as exponential growth (or decay) to solve for the doubling time: t = hours × ln(2) / ln(hCG2 ÷ hCG1). If the second value is lower than the first, the same formula returns a half-life instead of a doubling time, since hCG is falling rather than rising.
Interpreting your hCG numbers
- A single reading below 5 mIU/mL is typically read as negative, 5-25 mIU/mL as borderline (usually repeated in 48 hours), and above 25 mIU/mL as positive — but exact lab cutoffs can vary slightly by assay.
- In a normal, viable early intrauterine pregnancy, hCG usually doubles every 48-72 hours while levels are low, then rises more slowly after roughly 6-7 weeks of gestation.
- A rise slower than 48-72 hours, a flat reading, or a falling level does not automatically mean something is wrong, but it is a pattern clinicians follow up on — often with a repeat draw and an ultrasound — because it can also occur with an ectopic pregnancy or early pregnancy loss.
Educational tool, not a diagnosis
This calculator performs the arithmetic behind hCG interpretation for learning purposes; it cannot see your full history, ultrasound findings, or lab-specific reference ranges. Always discuss your actual results with your physician, midwife, or the lab that performed the test.