Gestation Probability Calculator

Cumulative birth probability · Skew-normal · μ = 280 d (GW 40) · σ = 13 d · α = −2

Last Menstrual Period (LMP)

Expected Due Date (mean)

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μ = 280 d · GW 40.0

σ = 13 d · α = −2

Profile

📚 Swedish registry (n=430k) · INTERGROWTH-21 · σ: US national birth data

⚠ Bimodal model — mixture of two Gaussians: term component (μ=280d, σ=11d, weight 78%) and a preterm-recurrence component (μ=245d ≈ GW35, σ=21d, weight 22%), reflecting a ~22% spontaneous preterm recurrence rate. IQR and median will appear shifted toward preterm. Not suitable for individual clinical decisions — consult an obstetrician. Sources: Mercer 1999, Dodd 2008.

μ — Mean (days)

Expected gestational age at birth

σ — Std deviation (days)

Spread of the distribution

α — Skew shape

0 = symmetric · negative = left skew (heavier preterm tail)

↔ Drag cursors A and B on the chart, or type a date in the fields below

Cursor A — Lower bound

Day 273

GW 39.0

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P(T ≤ A)

P ( A < T ≤ B )

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probability of delivering
between A and B

P ( T ≤ B | T > A )

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if still undelivered at A,
probability by B

Cursor B — Upper bound

Day 287

GW 41.0

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P(T ≤ B)

Late preterm (GW 32–37)

Early term (GW 37–39)

Full term (GW 39–41)

Late term (GW 41–42)

Post-term (GW 42+)

Zone P(T ≤ B | T > A)