Estimated Birth Window

When will she arrive?

Adjust the due date and clinical factors to personalise the probability distribution.

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When is your due date?

Enter your estimated due date to see a personalised birth probability calendar.

Statistical visualisation only — not medical advice.

— Peak day
— Expected day
— 50% window
— Shift vs baseline
Base distribution Still pregnant as of —

Toggle on to zero out past days and redistribute probability. Click any day to change the cutoff.

Passed
Today
Due date
<1%
1–2%
2–4%
4–6%
6–8%
8%+
How the calculation works

The calendar models the probability that birth occurs on each specific day, using a modified asymmetric Gaussian distribution centred on your due date — then adjusting it based on the clinical factors you configure.

1
Baseline curve. A bell curve is centred 3 days before the EDD (reflecting that spontaneous labour rarely waits for the exact due date). The left tail (early birth) is intentionally wider than the right — preterm risk is asymmetric. Hard cutoffs are applied below 37w and beyond 41w 6d.
2
Factor shifts. Each clinical factor contributes a mean shift (moving the peak earlier or later) and a spread shift (widening or narrowing the curve). These are summed across all active factors and applied to the baseline.
3
Taper after 41w. Probability decays exponentially beyond 41w 0d to reflect the increasing clinical likelihood of induction as post-dates risk rises.
4
Normalisation. All per-day values are scaled so that the total probability across the entire birth window sums to 100%.
5
Conditional mode. When "Still pregnant as of" is active, any day on or before the cutoff is zeroed out, and the remaining probabilities are renormalised — a simple application of Bayes' theorem.
P(day) ∝ Gaussian(day − EDD, μ + Σ shifts, σ ± Σ spreads)
μ baseline = −3 days  ·  σ left = 9 days  ·  σ right = 7 days
Final: P(day | not yet born) = P(day) / Σ P(future days)

Factor magnitudes are calibrated to published odds ratios and mean gestational age shifts from peer-reviewed studies (cited in each factor's detail panel). The model is a deliberate simplification — it captures the direction and rough magnitude of each risk factor, but does not model interaction effects between factors, and should not be used as a clinical tool.

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A gentle reminder — this is a statistical picture, not a prediction. Most babies arrive in their own time, and there is no "right" day. Please don't use this tool to set expectations, and try not to feel pressure or disappointment if things unfold differently. The waiting is hard, and that's completely normal.

If the uncertainty feels overwhelming, these resources may help:

Statistical visualisation only — not medical advice. Always follow the guidance of your midwife or OB/GYN.