Is my hCG rising as expected?
In early pregnancy hCG usually rises by at least 35% in 48 hours, though a slower rise can still be normal. Only your clinician can interpret the trend with your history.
hCG Change Calculator
Two-result mathematical comparison
Observed change
Next:
Keep the ordering clinician in the loop
The calculator can show
- Exact elapsed hours
- Observed change and ratio
- A mathematical 48-hour equivalent within the display window
- A mathematical doubling or halving time
The calculator cannot show
- Pregnancy location or viability
- Whether symptoms are safe
- Why the value changed
- Whether monitoring can stop
See the exact formula and display limits
Observed percentage change: (second value minus first value) divided by the first value, multiplied by 100.
48-hour equivalent: (ratio raised to 48 divided by elapsed hours, minus 1) multiplied by 100.
Doubling or halving time: elapsed hours multiplied by ln(2), divided by the absolute value of ln(ratio). Derived rates are displayed only when samples are 24 to 96 hours apart. The 24 to 96-hour window is a conservative display rule used by this tool to limit long-range extrapolation. It is not a clinical threshold.
Use this calculator to compare two hCG blood results taken at least 48 hours apart.
Everything behind the result
How this tool worksWhy two results matter more than one
How this tool works
Why two results matter more than one
In the first weeks hCG roughly doubles every 48 to 72 hours, but the normal range is wide and slows as levels rise.
The calculator works out the percentage change and doubling time between two blood results and shows where that sits against published ranges, without calling it normal or abnormal.
The full data tableEvery row this tool reads from.
Serial hCG: 48-hour equivalent and doubling time
What this calculator returns for a given rise across a given interval. The tool displays derived figures only when samples are 24 to 96 hours apart.
| Change between the two results | Over 24 hours | Over 48 hours | Over 72 hours | Over 96 hours |
|---|---|---|---|---|
| No change | No finite doubling time | No finite doubling time | No finite doubling time | No finite doubling time |
| +25% | 74.6 h doubling | 149.1 h doubling | 223.7 h doubling | 298.2 h doubling |
| +50% | 41.0 h doubling | 82.1 h doubling | 123.1 h doubling | 164.1 h doubling |
| Doubled (+100%) | 24.0 h doubling | 48.0 h doubling | 72.0 h doubling | 96.0 h doubling |
| Tripled (+200%) | 15.1 h doubling | 30.3 h doubling | 45.4 h doubling | 60.6 h doubling |
| -25% | 57.8 h halving | 115.7 h halving | 173.5 h halving | 231.3 h halving |
| Halved (-50%) | 24.0 h halving | 48.0 h halving | 72.0 h halving | 96.0 h halving |
These are mathematical transformations of two entered points. They do not determine pregnancy location, viability, why a value changed, or whether monitoring can stop. Serial hCG is interpreted with symptoms, pregnancy timing, ultrasound and the reason the test was ordered. Take both original pathology reports to the clinician who ordered them.
The full referenceEverything this tool draws on, in detail.Show the detail
Exact arithmetic with clinical boundaries
Separate what changed from what the results mean
The calculator reports the true interval, absolute change, percentage, ratio and eligible derived rates. It does not classify pregnancy location, viability or whether follow-up can stop.
Quick answers
What the arithmetic can show, and what needs clinical interpretation
What results can be entered?
Two exact positive numerical results from quantitative blood tests, with exact collection dates and times.
Why are exact times required?
The calculator uses the true elapsed interval rather than assuming the tests were exactly 48 hours apart.
Does a calculated doubling time show viability?
No. The maths cannot determine pregnancy location, viability or whether monitoring can stop.
When are derived rates withheld?
The 48-hour equivalent and doubling or halving time are withheld when the samples are less than 24 or more than 96 hours apart.
How to use the result
Use the mathematics without making a diagnosis
Observed change is not a viability label
The result describes two entered values and the exact time between them. It deliberately avoids week-by-week “normal” tables and good-or-bad labels because hCG ranges overlap widely and clinical interpretation depends on why testing was ordered, symptoms, ultrasound and the treating plan.
Collection time and laboratory method matter
A result collected 36 hours after the first sample should not be treated as though 48 hours elapsed. The calculator therefore uses exact timestamps. Results from the same pathology provider are generally easier to compare because assays and reporting methods can differ.
Treatment and urgent symptoms override the calculator
An hCG-containing trigger can contribute to measured hCG, and the calculator cannot separate medicine-derived from pregnancy-derived hormone. Severe or one-sided pain, shoulder-tip pain, heavy bleeding, significant dizziness, fainting, collapse or feeling very unwell requires medical assessment rather than waiting for another calculation.
Connected fertility tools
Continue from the blood-test arithmetic to the right next step
Each tool owns one task. Use only the step that matches the information available in the current cycle.
Related evidence and next steps
Use the page that owns the next pregnancy question
Complete hCG guide
Understand why serial results, symptoms and ultrasound are interpreted together.
Read Complete hCG guideAustralian evidence and resources
These sources provide general Australian or product context. They do not replace advice from a qualified clinician.
- hCG test
Healthdirect Australia Urine and blood hCG testing, early testing, sample dilution and following the exact test instructions.
- Ectopic pregnancy
Healthdirect Australia Pregnancy-related warning symptoms and when urgent assessment or triple zero (000) is needed.
- hCG levels
Healthdirect Australia The timing limits of hCG detection and why an early result may not settle a pregnancy question.
Frequently asked questions
- What is a normal hCG rise?
- At least 35% in 48 hours in early pregnancy, though many healthy pregnancies rise faster or slower.
- Can I use two home tests instead?
- No. Home tests show hCG is present, not how much.
- Does a slow rise mean miscarriage?
- Not on its own. Slow rises occur in ongoing pregnancies and in ectopic pregnancies, so a scan is usually needed.
- When should I go to emergency?
- Severe one-sided pain, heavy bleeding, shoulder-tip pain, dizziness or fainting. Call 000.
Australian evidence and resourcesShow sources
This is an educational timing aid, not a diagnosis.
Sources: Healthdirect Australia · RANZCOG
How hCG rises in early pregnancy and why ranges are wide.healthdirect.gov.au ↗
Warning symptoms and when urgent assessment is needed.healthdirect.gov.au ↗
What falling or plateauing hCG can indicate, and next steps.healthdirect.gov.au ↗
Clinical guidance on serial hCG interpretation.ranzcog.edu.au ↗