PonyatnoHow it's calculated
The time between tests, t, comes from the "hours between tests" field; if it is empty, it is the difference between the dates × 24 hours.
48-hour rise = ((hCG₂ / hCG₁)^(48 / t) − 1) × 100%. This brings results taken 24 or 72 hours apart to the same interval.
Doubling time = t × ln 2 / ln(hCG₂ / hCG₁), in hours and days. It is shown only when hCG rises: if it is flat or falling, there is no doubling.
The minimum expected 48-hour rise follows Barnhart et al. (2016) and depends on the first value: below 1500 IU/L — 49%, 1500 to 3000 IU/L — 40%, 3000 IU/L and above — 33%.
What to know
What the hCG rise shows
In the first weeks hCG rises quickly, and doctors look at the trend between two tests rather than a single number. Barnhart and colleagues established the 48-hour rise that is almost always seen in a developing pregnancy in the uterus. The higher the starting level, the slower the natural rise — so the minimum depends on the first value.
The old rule that "hCG doubles every 48 hours" is too strict: many normal pregnancies rise more slowly and develop well.
Limitations
A rise above the minimum doesn't tell where the pregnancy is — only an ultrasound shows that. A rise below the minimum doesn't prove anything on its own either. And once hCG is high, ultrasound tells more than the trend.
If you use dates without hours, a morning test versus an evening one adds a few percent of error. Different labs may report slightly different numbers.
What usually happens next
If the result raises questions, the doctor usually repeats hCG in 48 hours and arranges an ultrasound — two or three values plus the scan make the picture clear. Waiting can be stressful, but a single number rarely decides anything for good.
This calculator is a reference tool, not a diagnosis. Laboratory ranges differ, and the result depends on your health, medicines and other tests. Discuss it with your doctor.
FAQ
How fast should hCG rise?
Per Barnhart (2016), with a starting level below 1500 IU/L the minimum 48-hour rise is about 49%, from 1500 to 3000 it is 40%, above that 33%. Many pregnancies rise faster; doubling every 48 hours isn't required.
My hCG is rising slowly — is that bad?
Not necessarily. A slower rise is also seen in normal pregnancies. It's a reason to repeat the test and have an ultrasound; the conclusions are your doctor's to draw.
When should hCG be repeated?
Usually after 48 hours, in the same lab. The calculator handles other gaps too, but a very short one (under a day) gives an unreliable result.
Are mIU/mL and IU/L different?
No, they are the same: 1 mIU/mL = 1 IU/L. Enter the number as it is.
Can hCG show that the pregnancy is in the uterus?
No. The hCG rise doesn't show where the pregnancy is — only an ultrasound can.
Sources
Formulas and cut-offs follow clinical guidelines — see sources below.
- Barnhart KT et al. Early trajectory of human chorionic gonadotropin in a viable intrauterine pregnancy. Obstet Gynecol. 2016;128(3):504–511.