Enter two quantitative hCG blood-test results and their collection times. See the change between the tests, the equivalent 48-hour change, and an estimated doubling or halving time.
Your result
How to use this calculator
- Enter the collection date, time, and quantitative serum hCG result for the first blood test.
- Enter the later test using the same units and, if possible, the same laboratory.
- Select Calculate change.
- Read the observed change first. The mathematical 48-hour estimate shows the equivalent change if the same two-test rate is expressed over 48 hours. The doubling or halving time summarizes that rate.
Choose Advanced if you have three or more results or want an interval chart and projection. This calculator is educational and cannot diagnose pregnancy viability or location or replace medical care.
Frequently asked questions
What does the mathematical 48-hour estimate mean?
It applies one constant exponential rate between the two measurements and expresses that rate over 48 hours. It is a mathematical comparison, not a prediction that hCG will continue changing at that rate.
Can the calculator use tests that are not 48 hours apart?
Yes. It can calculate the observed change for any positive interval, but that does not mean every testing interval is equally useful clinically. NICE advises clinicians managing a pregnancy of unknown location to take two serum hCG measurements as near as possible to 48 hours apart, but not earlier. The Barnhart two-day research comparison is shown only when the entered interval is exactly 48 hours.
What does doubling or halving time mean?
It summarizes the rate between these two measurements. It does not predict the next result or confirm pregnancy viability or location.
What do Rising, Falling, and Unchanged mean?
They describe only the mathematical direction between the two results. They are not diagnostic labels.
Can I mix results from different laboratories?
Use the same laboratory when possible because assays can differ. Consistent testing reduces noise in short-interval comparisons.
When should I use Advanced?
Use the multi-draw hCG calculator for three or more results, interval comparisons, a chart, or a projection. Its headline result uses the latest valid interval.
Methodology and sources
What the calculator reports
- The observed percentage change between the two measured hCG results.
- The time between the tests.
- A mathematical 48-hour estimate and multiplier based on one constant exponential rate between the tests.
- An estimated doubling time for a rise or halving time for a fall.
- A descriptive Rising, Falling, or Unchanged label.
Calculations
Let v1 be the first value, v2 the second value, and dt the elapsed hours.
- Observed change:
(v2 / v1 - 1) × 100. - Hourly rate:
k = ln(v2 / v1) / dt. - 48-hour multiplier:
exp(k × 48). - Doubling time for a rise:
ln(2) / k. Halving time for a fall:ln(2) / |k|.
The mathematical 48-hour estimate and doubling or halving time describe the rate between these two measurements. They do not predict that the same rate will continue.
Starting-level research comparison
For a rising result measured over exactly 48 hours, the calculator may show a lower-end research comparison based on the first hCG value. Barnhart and colleagues studied 285 women with first-trimester pain, with or without bleeding, and a pregnancy of unknown location whose pregnancies were later confirmed as ongoing intrauterine pregnancies. The modeled first-percentile two-day rise was about 49% for a starting hCG of 1,500 or less, 40% above 1,500 through 3,000, and 33% above 3,000 through 10,000. These are lower-end comparisons for that study population, not typical averages, normal ranges, or diagnostic cutoffs. The comparison is withheld when the interval is not exactly 48 hours or the first result is above 10,000.
Scope and limits
- Intervals under 24 hours receive a warning because rescaling to 48 hours can magnify small measurement differences. Intervals over 96 hours receive a warning because the biological rate may have changed.
- Different laboratory assays can produce small differences. Use the same laboratory when possible.
- Serial hCG cannot by itself confirm pregnancy viability or location. Symptoms, repeat testing, and ultrasound findings guide clinical care.
Sources
- Barnhart et al. Differences in Serum Human Chorionic Gonadotropin Rise in Early Pregnancy by Race and Value at Presentation. Obstetrics and Gynecology. 2016.
- NICE. Ectopic pregnancy and miscarriage: diagnosis and initial management (NG126). Diagnosis of viable intrauterine pregnancy and tubal ectopic pregnancy.
- NICE. Symptoms and signs of ectopic pregnancy and initial assessment.
- Mullany K, Minneci M, Monjazeb R, Coiado OC. Overview of ectopic pregnancy diagnosis, management, and innovation. Women's Health. 2023.
- StatPearls. Human Chorionic Gonadotropin.
Support if you have experienced a loss
Reading about miscarriage, stillbirth, or infant loss can be difficult. If you have experienced a loss, you do not have to use this calculator or read the statistics. You may find it more helpful to talk with your care team or a support organization.
In the United States, groups such as:
offer information and bereavement support. If you are elsewhere, your local health service or a pregnancy and baby loss charity may be able to guide you to support in your region.