By Zencalcy Editorial TeamLast reviewed About us
Beta-hCG usually rises rapidly in early pregnancy, and the speed of that rise can add context to a clinical picture. This calculator uses two hCG values and the days between them to estimate doubling time in hours and the equivalent percentage rise per 48 hours. A single pair of results cannot confirm a healthy pregnancy, and interpretation belongs with a clinician and ultrasound.
Doubling time and 48-hour rise from two hCG values.
Doubling time (days) = days × ln(2) ÷ ln(hCG₂ ÷ hCG₁) Rise per 48 h = ((hCG₂ ÷ hCG₁)^(2 ÷ days) − 1) × 100
The formula assumes exponential growth between the two samples. Doubling time lengthens naturally as hCG rises, so absolute values and the trend matter more than one ratio. Results from different laboratories or assays should not be compared directly. Slow or falling values can occur in miscarriage or ectopic pregnancy, but they can also have other explanations.
| Symbol | Meaning |
|---|---|
| hCG₁ | First beta-hCG value in mIU/mL. |
| hCG₂ | Second beta-hCG value in mIU/mL. |
| Days | Number of days between the two samples. |
The ratio is 2, so doubling time = 2 × ln(2) ÷ ln(2) = 2 days, or 48 hours. The 48-hour rise is (2^(2/2) − 1) × 100 = 100%. A rise from 100 to 300 over 3 days gives a doubling time of about 45.4 hours and a 48-hour rise of about 108%.
Human chorionic gonadotropin is produced by the developing placenta and rises quickly after implantation. The slope of that rise is informative, but it is not a test with a single pass-or-fail threshold. Values can start low and still progress, and a slower rise can sometimes have a benign explanation. Conversely, a reassuring ratio cannot exclude an ectopic pregnancy or confirm viability. Ultrasound, symptoms and the full clinical picture are essential. The calculation here simply turns two numbers and a time interval into a rate, so the trend can be discussed in consistent units. Bring the original lab reports, including the units and reference ranges, to your appointment.
Continue with the Pregnancy Weight Gain Calculator, Gestational Age Calculator, or the Conception Date Calculator — or browse more gynecology & pregnancy tools and all health calculators.
Also see the Ovulation Calculator and the Pregnancy Due Date Calculator.
There is no single normal value. Doubling time is faster at low hCG levels and slows as the level rises, and normal ranges are wide. Clinicians interpret the trend alongside the starting value, symptoms and ultrasound rather than applying one cutoff to every pregnancy.
Pregnancy dating and fertility estimates assume regular cycles and typical ovulation. They cannot confirm pregnancy, viability or gestational age, and they do not replace ultrasound or prenatal care. Seek urgent care for pain, bleeding or reduced fetal movement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
Content updated: September 2026