hCG Levels & Doubling Time Calculator (with Chart)

How this estimate works

1. Sort & standardize. We sort your hCG results by date/time (earliest → latest). All values are assumed in mIU/mL from the same lab assay.
2. Compute growth rate and doubling time. If you enter 2 points, we treat growth as exponential and use:
   k = ln(h2 / h1) / Δt_days, then doubling_time = ln(2) / k (if k > 0). With 3+ points, we fit a straight line to ln(hCG) vs time (least squares) and take its slope as k. Negative k implies a decline (half-life = ln(2)/|k|).
3. Project to a future date. Using the fitted rate k, predicted hCG at time t is h(t) = h_last * exp(k * Δt_days).
4. Typical rise check (48 h windows). Early viable intrauterine pregnancies usually rise quickly. As a conservative lower bound, we flag rises below ~35% over 48 h as “below the lowest expected range.” Expected 48 h rises also depend on the starting hCG: approx. ≥49% if <1500, ≥40% if 1500–2999, ≥33% if ≥3000 mIU/mL.
5. Gestational-age context. Rise slows with advancing gestational age and higher absolute hCG. Very early (roughly before ~6 weeks GA), doubling is often ~48–72 h; beyond that it lengthens and eventually plateaus near 8–10 weeks.
6. IVF/FET context (if provided). When a transfer date is entered, we map to GA using ART conventions (use embryo age at transfer): for a day-5 embryo, EDD = transfer date + 261 days; day-3 = +263 days. GA at a blood draw follows from that dating.

Formulas (copyable)

Two-point growth:
  k = ln(h2 / h1) / Δt_days
  Doubling time (if k > 0) = ln(2) / k
  Half-life (if k < 0)     = ln(2) / |k|

Multi-point fit (least squares on ln(h)):
  Fit y = a + k * t, where y = ln(hCG), t in days since first draw.
  Use fitted slope k for doubling/half-life and projections.

Projection from last value h_last at time t_last to t_future:
  h_pred = h_last * exp(k * (t_future - t_last))

Typical 48 h rise thresholds

As a conservative lower bound across early viable pregnancies, rises < ~35% in 48 h are concerning, but overlap exists and clinical context/ultrasound matters.

Interpretation notes

• One unusual value can distort the slope. When 3+ points are available, the log-linear fit reduces noise compared with a single 48 h ratio.
• Doubling time varies with GA and absolute hCG; a slower rise later can be normal. Ultrasound (typically from ~6–7 weeks GA) is definitive for location and viability.
• Declines or very slow rises warrant clinical follow-up for pregnancy of unknown location, failing IUP, or ectopic risk according to local protocols.

Limits & safety

• Educational estimate; not diagnostic. Always interpret with symptoms and ultrasound findings.
• Different assays/labs can yield different numbers; compare like with like where possible.
• Do not use to time or dose medications.

Sources

• Barnhart KT et al. Obstet Gynecol. 2004: baseline “minimal rise” work in symptomatic early pregnancy.
• Seeber BE, Barnhart KT. Fertil Steril. 2006: redefined curves; viable gestations may rise as slowly as ~35% in 48 h.
• Perinatology.com: expected 48 h rises vs starting hCG (≥49%, ≥40%, ≥33% tiers).
• Fertil Steril. 2016: hCG doubling time lengthens with GA and higher concentrations.
• ACOG Committee Opinion No. 700 (2017; reaffirmed): ART/IVF dating conventions (e.g., day-5 = +261 d; day-3 = +263 d).
• AJOG classic data: hCG peaks approx. 8–10 weeks GA before declining/plateauing.