hCG Doubling Time Calculator (with Chart)

How the doubling time is calculated

• Sort and clean: entered draws are sorted by date and time. Only rows with a valid date/time and a positive hCG value are used.
• Log-rise model: early hCG change is treated as exponential. The calculator fits a line to ln(hCG) versus hours since your first draw.
• Two draws: slope k equals log change divided by the time gap. Doubling time = ln(2) ÷ k hours.
• Three or more draws: a least-squares line is fit to all points, then doubling time is ln(2) ÷ k.
• Declining or flat: if k ≤ 0 the tool shows a halving time when k < 0, or “no rise” when the slope is zero within rounding.

“Typical” rise bands by stage

• No GA given: a general early-pregnancy band of about 48–72 h is used for comparison.
• With GA context: when gestational age is available the comparison band widens with advancing GA, reflecting slower rises at higher hCG:
  • ≤ ~6 weeks (≤ 42 days): 48–72 h
  • ~6–7 weeks (≤ 49 days): 72–96 h
  • > ~7 weeks: 96–144 h
• Badge: the headline badge reads Declining when k ≤ 0, otherwise classifies as Faster than typical, Within typical, or Slower than typical against the stage-matched band.

Projection and chart

• Projection: the chosen “Project to” date/time is predicted from the fitted slope using the last measured value as the start. If empty, projection defaults to +48 h after the last draw.
• Chart: the Y-axis is logarithmic (IU/L). The X-axis is hours since the first draw. A shaded envelope shows the stage-appropriate “typical” rise from the first point.
• Progress bar: the bar maps doubling times from about 24–144 h to a 0–100% fill to give quick visual context.

Data entry tips

• Use the exact report date/time for each draw and the same laboratory when possible.
• Intervals of 24–48 hours are most informative. Very short or very long gaps are harder to interpret.
• Later in the first trimester, rises slow naturally; a longer doubling time can still be normal.

Limitations

• Serial hCG trends should be interpreted with symptoms and, when appropriate, ultrasound.
• Assay variability and biological variation can shift values between labs and over time.
• Atypical patterns can occur with ectopic pregnancy, miscarriage, or trophoblastic disease; this calculator does not diagnose these conditions.