What this calculator does
hCG, human chorionic gonadotropin, is the hormone measured in early pregnancy blood tests. Because it normally rises quickly in the first weeks, clinicians sometimes look at how fast it is doubling between two readings taken a few days apart, as one of several pieces of information used to gauge how a very early pregnancy is progressing.
This calculator only does the arithmetic: given two hCG readings and the number of days between them, it works out the doubling time implied by that change. It does not interpret the result, and it is not a substitute for a doctor reviewing the readings alongside ultrasound findings, symptoms and the rest of the clinical picture, which is where the real interpretation happens.
The formula
Doubling time is the number of days between readings, multiplied by the natural log of 2, divided by the natural log of the second reading divided by the first. This is the standard exponential-growth doubling-time formula applied to the two hCG values.
| Term | Meaning |
|---|---|
| hCG | Human chorionic gonadotropin, the hormone measured by a quantitative blood pregnancy test, in mIU/mL. |
| Doubling time | How many days it would take the hCG level to double, projected from the rate of change between the two readings given. |
| Percentage change | The overall rise (or fall) between the two readings, shown alongside the doubling time for context. |
The inputs explained
| Field | What to enter |
|---|---|
| First hCG reading (mIU/mL) | The earlier of the two hCG readings, in mIU/mL, as reported on the lab result. |
| Second hCG reading (mIU/mL) | The later of the two hCG readings, in mIU/mL. |
| Days between the two readings | The number of days between when the two blood samples were taken. |
When to use it
Reviewing two early readings ahead of a doctor’s appointment
Working out what doubling time two readings imply can help make sense of the numbers before discussing them with a doctor, who will weigh them alongside everything else known about the pregnancy.
Readings taken close together
A short gap of one or two days between tests is common in early monitoring; the calculator handles any gap you enter, though very short gaps can make the implied doubling time swing a lot for a small change in the reading.
A reading that has not risen
If the second reading is not higher than the first, a doubling time cannot be calculated at all, and the calculator says so directly rather than returning a misleading number.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How doubling time changes with the size of the rise, over a fixed 2-day gap
A fixed first reading and a fixed 2-day gap, against a range of second readings.
| Second reading | Estimated doubling time | Overall change over that period |
|---|---|---|
| 150 mIU/mL | 3.4 days | 50.0% |
| 175 mIU/mL | 2.5 days | 75.0% |
| 200 mIU/mL | 2.0 days | 100.0% |
| 250 mIU/mL | 1.5 days | 150.0% |
| 300 mIU/mL | 1.3 days | 200.0% |
| 400 mIU/mL | 1.0 days | 300.0% |
Questions
Is a specific doubling time normal or not normal?
This calculator does not say, on purpose. Normal hCG doubling times vary widely between healthy pregnancies and change as the pregnancy progresses, so a single figure needs a doctor to interpret it against the full picture, not a fixed cutoff applied on its own.
What if the second reading is lower than the first?
A falling hCG level cannot produce a doubling time, since the level is not doubling at all. A falling or flat reading between two tests is something to raise with a doctor promptly rather than something to interpret using any calculator.
Can this calculator tell me if a pregnancy is viable?
No. hCG trends are only one input doctors use alongside ultrasound findings, symptoms and other test results. This tool only performs the doubling-time arithmetic; it makes no assessment of viability and is not a diagnostic tool.
Why does a very short gap between readings make the result swing so much?
With only a day or two between tests, a small difference in the reading translates into a large swing in the calculated doubling time, since the formula is dividing by a very small time interval. Longer gaps between readings tend to give a steadier estimate.
For a general read on early pregnancy timing by dates rather than hormone levels, see the estimated conception date calculator. For anything about these readings themselves, that conversation belongs with a doctor.