How this calculator works
The observed mode fits a constant exponential rate between two positive laboratory measurements. Reverse modes solve a level or sample interval from a user-specified doubling or halving time; they are mathematical scenarios, not predicted clinical results. It describes the observed interval and extrapolates that rate to 24 and 48 hours; real hCG does not necessarily keep that rate. Use the same units and comparable assays. Unchanged levels have no finite doubling time, while falling levels have a halving time. The trend cannot confirm pregnancy location or viability, exclude ectopic pregnancy or determine treatment. The result is an estimate for information only and is not medical advice.
Formula
k = ln(second/first) / elapsed hours. Doubling hours = ln(2)/k when k > 0; halving hours = -ln(2)/k when k < 0. Change at t hours = exp(k t) - 1.
Worked example
A rise from 100 to 200 in 48 hours has a 48-hour doubling time, 100% observed change and about 41.42% equivalent 24-hour change. A fall from 200 to 100 has a 48-hour halving time.
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Frequently asked questions
Must hCG double every 48 hours?
No. The expected course depends on timing, starting value and the clinical circumstances. This tool supplies no universal normal threshold.
Are IU/L and mIU/mL equivalent?
Yes, they are numerically equivalent. Other unit scales must be converted consistently.
What if I have pain, fainting or heavy bleeding?
Seek urgent medical assessment. A reassuring-looking hCG calculation cannot exclude an emergency.