# What is my hCG doubling time?

Computes the hCG doubling time and percent rise between two blood tests, the rise scaled to 48 hours, and compares it with the smallest 2-day rise seen in ongoing pregnancies (Barnhart 2016).

- Page: https://www.acalculator.org/health/hcg-calculator
- JSON spec: https://www.acalculator.org/health/hcg-calculator.json
- Version: 7d5553d0639e

## Default answer

Example with the default inputs (First hCG (mIU/mL) 1,000, Second hCG (mIU/mL) 1,800, Hours between tests 48): From 1,000 to 1,800 mIU/mL in 48 hours: the doubling time is 56.6 hours, a 80% change over 48 hours.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| h1 | First hCG (mIU/mL) | The first blood hCG result, in mIU/mL (the same as IU/L). |
| h2 | Second hCG (mIU/mL) | The later blood hCG result, from the same lab if you can. |
| hours | Hours between tests | How many hours passed between the two blood draws; 48 for two days. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| time | Doubling time (hours) | Hours for hCG to double at this rate; when it falls, hours for it to halve. |
| trend | Direction | Whether hCG rose (doubling time) or fell (halving time). |
| days | Doubling time (days) | The same time in days. |
| change | Change between tests | Second ÷ first − 1, in percent. |
| rise48 | Change over 48 hours | The change scaled to exactly 48 hours, assuming a steady exponential rate. |
| minimum | Smallest 2-day rise in Barnhart 2016 | The 1st-percentile 2-day rise in ongoing intrauterine pregnancies, by the first level. |
| compare | Compared with that rise | Whether the 48-hour change is at least that rise. |

## Method

Doubling time = hours × ln 2 ÷ ln(second ÷ first) (halving time when it falls: hours × ln 2 ÷ |ln(second ÷ first)|); change = (second ÷ first − 1) × 100; 48-hour change = ((second ÷ first)^(48 ÷ hours) − 1) × 100.

## Assumptions

- hCG changes at a steady exponential rate between the two tests.
- Barnhart 2016 studied 285 women with an ongoing intrauterine pregnancy who had bleeding or pain in the first trimester; its smallest 2-day rise is 49% for a first value below 1,500 mIU/mL, 40% from 1,500 to 3,000, and 33% above 3,000.
- This is information, not a diagnosis. A single slow rise does not rule out a healthy pregnancy, and a normal rise does not rule out an ectopic one.

## Worked examples

1. h1 = 1,000, h2 = 2,000, hours = 48 gives time = 48, days = 2, change = 100%, rise48 = 100%, minimum = 49%, compare = At or above it. Source: Barnhart KT et al., Differences in Serum Human Chorionic Gonadotropin Rise in Early Pregnancy by Race and Value at Presentation, Obstet Gynecol 2016;128(3):504-511, doi:10.1097/AOG.0000000000001568 (2-day minimal rise 49% below 1,500 mIU/mL). https://pubmed.ncbi.nlm.nih.gov/27500326/.
2. h1 = 1,000, h2 = 1,800, hours = 48 gives time = 56.60398, change = 80%, rise48 = 80%. Source: Barnhart KT et al., Obstet Gynecol 2016;128(3):504-511, doi:10.1097/AOG.0000000000001568. https://pubmed.ncbi.nlm.nih.gov/27500326/.
3. h1 = 2,000, h2 = 2,500, hours = 72 gives change = 25%, rise48 = 16.039721%, minimum = 40%, compare = Below it, time = 223.652428. Source: Barnhart KT et al., Obstet Gynecol 2016;128(3):504-511, doi:10.1097/AOG.0000000000001568 (2-day minimal rise 40% for 1,500 to 3,000 mIU/mL). https://pubmed.ncbi.nlm.nih.gov/27500326/.
4. h1 = 5,000, h2 = 2,500, hours = 48 gives time = 48, change = -50%, trend = Falling: halving time, minimum = 33%, compare = Below it. Source: Barnhart KT et al., Obstet Gynecol 2016;128(3):504-511, doi:10.1097/AOG.0000000000001568 (2-day minimal rise 33% above 3,000 mIU/mL). https://pubmed.ncbi.nlm.nih.gov/27500326/.

## FAQ

### How do I calculate hCG doubling time?

Doubling time = hours between tests × ln 2 ÷ ln(second result ÷ first result). From 1,000 to 1,800 mIU/mL in 48 hours: 48 × 0.6931 ÷ 0.5878 = 56.6 hours.

### How much should hCG rise in 48 hours?

In Barnhart 2016, a study of 285 women with an ongoing intrauterine pregnancy who had bleeding or pain, the smallest 2-day rise (1st percentile) was 49% when the first value was below 1,500 mIU/mL, 40% from 1,500 to 3,000, and 33% above 3,000. Most rose faster. Only a clinician can interpret your results.

### Does hCG always double every 48 hours?

No. A doubling every 2 to 3 days is common early on, but the rise slows as the level climbs, which is why Barnhart’s minimum rise is lower for higher first values. A slower rise is not a diagnosis on its own.

### What if my tests were not 48 hours apart?

Type the real number of hours. The page assumes a steady exponential rate and scales the change to 48 hours: a 25% rise over 72 hours is 1.25^(48 ÷ 72) − 1 = 16.0% over 48 hours.

### What does a falling hCG mean?

The page shows a halving time instead of a doubling time. Falling hCG has several causes, and only your clinician can say what it means for you.

### Can I use home urine test results?

No. Home tests show only whether hCG is above a threshold. The page needs quantitative blood (serum) results in mIU/mL, ideally from the same lab.

## Sources

- Barnhart KT, Guo W, Cary MS, Morse CB, Chung K, Takacs P, Senapati S, Sammel MD. Differences in Serum Human Chorionic Gonadotropin Rise in Early Pregnancy by Race and Value at Presentation. Obstet Gynecol. 2016;128(3):504-511. doi:10.1097/AOG.0000000000001568. https://pubmed.ncbi.nlm.nih.gov/27500326/ (abstract read 2026-10-01)