Enter two hCG results and the hours between them to see the doubling time and percent rise, with the formula worked out.
Estimates only, not medical advice.
In early pregnancy hCG typically rises quickly, often roughly doubling every 48 to 72 hours. The calculator solves for doubling time using the ratio of your two readings and the hours between them: doubling time equals hours times the natural log of 2, divided by the natural log of the second reading over the first.
Take a first reading of 100 mIU/mL and a second reading of 220 mIU/mL 48 hours later, the calculator's own default example. The ratio of the two readings is 2.2. The natural log of 2.2 is about 0.788, and the natural log of 2 is about 0.693. Dividing 48 times 0.693 by 0.788 gives a doubling time of roughly 42 hours, faster than the typical 48 to 72 hour range, which the pattern field would flag as within the typical early range since it falls under 72 hours.
| Doubling time | How this calculator labels it |
|---|---|
| 72 hours or less | Within the typical early range |
| More than 72 hours | Slower than typical, discuss with your provider |
| Second reading not higher than first | Levels not increasing, ask your provider |
Doubling time is not constant across a pregnancy. It naturally lengthens as hCG rises into the thousands and tens of thousands, so a result that looks slow late in the first trimester is not directly comparable to the same doubling time measured on day 20. Lab to lab differences, the exact timing of blood draws, and normal biological variation also mean a single pair of numbers is a data point, not a verdict. A wide range of doubling times shows up in ultimately healthy pregnancies, and a slower than expected rise on its own is not diagnostic in either direction. Your provider interprets any single doubling time result alongside your symptoms, history, and, when needed, additional testing.
It uses the ratio between your two results and the hours between tests to solve for how long it would take the level to double at that same rate of increase.
Not necessarily. Doubling time naturally slows as hCG rises higher, and a wide range of results can appear in healthy early pregnancies. Your provider interprets the trend alongside your history and any symptoms.
Lab variation, testing too close together, or levels that are naturally slowing near their peak can all produce a smaller than expected rise. A single reading rarely tells the whole story.
That is a question for your provider, who will weigh your specific numbers, symptoms, and history before recommending a retest interval.