hCG 48-Hour Change Calculator: Percent Rise Between Two Tests

Enter two serum hCG tests. The tool shows the observed change, normalises it to a 48-hour rise, and labels the pattern.

Draw 1
Draw 2
Advanced

What this tool calculates

  • Observed percent change between two hCG tests taken at different times.
  • Change normalised to a 48 hour interval using an exponential model so results are comparable even if the gap is not exactly 48 hours.
  • A simple label band for context: Falling, Plateau, Slow rise, Within typical.

Formulas

  • Let v1 and v2 be the two hCG values, and dt be the time between draws in hours.
  • Observed change = (v2 − v1) / v1.
  • Growth rate per hour k = ln(v2 / v1) / dt.
  • 48 hour ratio r48 = exp(k × 48). The 48 hour normalised change = r48 − 1.

Label bands used in the card

  • Falling: r48 < 0.99.
  • Plateau: 0.99 ≤ r48 < 1.10.
  • Slow rise: 1.10 ≤ r48 < 1.66.
  • Within typical: r48 ≥ 1.66.

Published guidance notes that in very early viable intrauterine pregnancies, the minimal expected rise over 48 hours can be lower than a doubling, and depends on the starting hCG level. Services use local protocols when interpreting serial values.

Scope and limits

  • Educational context for early pregnancy hCG trends. Not a diagnostic test.
  • Interpretation depends on clinical findings, ultrasound, and baseline hCG.
  • Assays from different labs may vary slightly. Use the same lab where possible.

Sources

  • NICE guideline NG126, Ectopic pregnancy and miscarriage: diagnosis and initial management. Serial hCG guidance and trends. nice.org.uk/guidance/ng126
  • StatPearls, Human chorionic gonadotropin (hCG) overview. Typical early pregnancy rise and doubling time context. ncbi.nlm.nih.gov/books/NBK532950/

Last updated: 6 Oct 2025

How to use this calculator

  1. Enter Draw 1: add the exact date/time and the serum hCG value.
  2. Enter Draw 2: add the later date/time and value from the same lab if possible.
  3. Press Calculate: view the observed percent change and a 48-hour normalised change with a plain label.
  4. Need more context? open the full hCG Doubling Time calculator.

Results are estimates only. Use with clinical judgement and formal follow-up.

Last updated: October 2025

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.