Beta hCG Calculator for Twins/Triplets

What this tool does

• Calculates the doubling time of serum hCG from two or more dated blood draws and classifies the rise as faster than typical / within typical / slower than typical for twins or triplets.
• Fits a growth curve on the natural‐log of hCG versus hours to estimate a continuous rise rate, then reports the implied doubling time and an optional projection to a future date/time.

Inputs

• At least two draws with valid date–time and hCG (IU/L).
• Pregnancy type toggle: Twins or Triplets.
• Optional projection timestamp; if blank, the tool projects +48 h from the latest draw.

Computation steps

• Preprocess. Sort draws by time. Convert each time to hours since the first draw: x_i = (t_i − t₀)/3600000. Take natural log of each value: y_i = ln(hCG_i).
• Slope estimation.
  • Exactly two points: slope k = (y₂ − y₁) / (x₂ − x₁); intercept a = y₁ − k·x₁.
  • More than two points: ordinary least squares line on (x,y) blended toward the most-recent interval: k̂ = 0.7·k_OLS + 0.3·k_last, â = 0.7·a_OLS + 0.3·a_last. This gives stability with multiple draws while staying sensitive to the latest trend.
• Doubling / halving time. If k > 0, doubling time T₂ = ln(2)/k. If k < 0, report a halving time T½ = ln(2)/|k| and classify as declining.
• Projection. From the latest observed value V_last at time x_last, the value at a future offset Δh hours is V̂ = V_last · e^{k·Δh}.
• Observed change (first → second). If two earliest points are available, show percent change ((V₂ − V₁)/V₁) over the actual hour gap.

Classification bands (multiples‐adjusted)

• Baseline “typical early rise” band for singletons is commonly presented as roughly 48–72 h for doubling very early in gestation. Multiples often have higher absolute hCG but overlapping rise rates.
• To reflect slightly faster rises sometimes seen in multiples while staying conservative, the tool narrows this band by a small factor:
  • Twins: band = 0.95 × [48,72] → [46,68] h (rounded to practical bounds).
  • Triplets: band = 0.90 × [48,72] → [43,65] h (rounded to practical bounds).
  A measured T₂ below the lower edge is labelled faster than typical; within the band typical; above the upper edge slower than typical. Declining trends are labelled separately.
• The UI also shows the expected 48-hour change band implied by the adjusted range: Δ48_min = 2^{48/upper} − 1, Δ48_max = 2^{48/lower} − 1.

Display logic

• Headline: Doubling time in hours (and days), with a badge showing the classification for the selected pregnancy type.
• Bar: Fills more as doubling time shortens (bounded between 24–144 h for visualization).
• Extras: Projected 48-hour change versus the typical band; observed percent change between the first and second draws and their hour gap.
• Chart: Log-scaled hCG (IU/L) against hours since the first draw. Plots (a) the fitted exponential, (b) the shaded “typical” band for the chosen type, (c) your measured points, and (d) the projection marker.

Scope and limits

• This is an educational tool. hCG behavior varies widely and depends on lab methods, sampling time, assay calibration, and clinical context.
• Rise rates change across gestation; very high hCG or late-first-trimester values may rise more slowly even in healthy pregnancies.
• Multiples often show higher absolute hCG, but doubling dynamics still overlap singletons; small “faster” adjustments are therefore used to avoid overcalling.
• Do not use for diagnosis (e.g., ectopic, nonviable, or chorionicity decisions). Always interpret alongside ultrasound, symptoms, and clinician guidance.

Last updated: 16 Oct 2025