Enter two hCG results and the date and time for each draw. This calculator shows the percent change over your actual time gap and the change normalised to 48 hours.
How to use this calculator
- Enter Draw 1: add the date/time and the serum hCG value.
- Enter Draw 2: add the later date/time and the new value (same lab if possible).
- Press Calculate: you will see the observed percent change over your actual time gap.
- Check the 48-hour result: this normalizes your change to 48 hours, even if your tests were not exactly 48 hours apart. For doubling time, use the full hCG Doubling Time calculator.
This is an educational estimate. It does not diagnose pregnancy viability or replace medical care.
Frequently asked questions
What does the 48-hour normalised change mean?
It rescales your two results to a 48-hour window using an exponential model. This allows comparison to typical early pregnancy rises.
How is the label decided?
The tool classifies the 48-hour change as Falling, Plateau, Slow, or Within typical. It is a simple rule set for education and is not diagnostic.
Do I need exactly 48 hours between tests?
No. Any positive gap works. The tool adjusts to 48 hours mathematically so you can compare different intervals fairly.
Can I mix results from different labs?
Try to use the same lab. Assays can differ slightly. Consistency reduces noise in short-interval comparisons.
Should I use the doubling time tool instead?
Use the doubling time tool when you have three or more draws or want a fitted slope and projection. This 48-hour tool compares two tests only.
Methodology and sources
What this calculator reports
- The observed percent change between two hCG results.
- A 48-hour normalized percent change, so two tests can be compared even if the time gap is not exactly 48 hours.
- A simple label based on the 48-hour ratio: Falling, Plateau, Slow, Within typical.
Inputs and time gap
- The tool reads the two date/time fields and computes the time gap in hours (
dt). - It requires
dt > 0(Draw 2 must be after Draw 1) and both hCG values must be positive.
Calculations
Let v1 be the first hCG value, v2 the second value, and dt the time between draws in hours.
- Observed change (fraction):
(v2 - v1) / v1. - Exponential growth rate per hour:
k = ln(v2 / v1) / dt. - 48-hour ratio:
r48 = exp(k * 48). - 48-hour normalized change (fraction):
r48 - 1.
Display rounding follows the on-page results: percent values show to 1 decimal place (trailing .0 removed), the 48-hour ratio shows to 2 decimals, and the time gap shows to 1 decimal hour (or whole hours for long gaps).
Label bands used
These bands are built into the tool to give quick context for the 48-hour ratio (r48).
- Falling:
r48 < 0.99. - Plateau:
0.99 ≤ r48 < 1.10. - Slow:
1.10 ≤ r48 < 1.66. - Within typical:
r48 ≥ 1.66.
Clinicians often interpret serial hCG alongside symptoms and ultrasound. Published thresholds for a "minimal expected rise" vary by the starting hCG level and the clinical setting. This tool does not adjust the bands by starting level.
Scope and limits
- This is for understanding trends in serial measurements. It is not a diagnosis.
- Different labs and assay methods can produce small differences. Using the same lab can reduce variation.
- A single pair of values can be misleading. Clinical care often relies on repeat testing and ultrasound timing.
Sources
- NICE. Ectopic pregnancy and miscarriage: diagnosis and initial management (NG126). Recommendations include guidance on interpreting serial hCG in pregnancy of unknown location. https://www.nice.org.uk/guidance/ng126/chapter/recommendations
- Bansal A, Patil M, Rockall A, Jurkovic D. Overview of ectopic pregnancy diagnosis, management, and innovation. Includes a summary of expected 48-hour hCG changes in viable intrauterine pregnancy and thresholds used in practice. https://pmc.ncbi.nlm.nih.gov/articles/PMC10071153/
- StatPearls (NCBI Bookshelf). Human Chorionic Gonadotropin. https://www.ncbi.nlm.nih.gov/books/NBK532950/
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.