Enter your due date and clinical profile to generate a personalized, day-by-day chart showing when spontaneous labor is most likely to begin — based on published evidence.
Please read carefully before using this calculator. You must acknowledge these limitations to proceed.
This tool estimates the statistical probability of spontaneous labor onset based on population-level data combined from multiple published studies. Risk factor adjustments are derived from individual studies with differing populations, designs, endpoints, and time periods. Combining these effect sizes introduces methodological uncertainty. The estimates are illustrative of population patterns — not a precision forecast for any individual pregnancy.
This calculator does not replace clinical assessment, obstetric ultrasound, fetal monitoring, or the judgment of a qualified obstetric provider. Delivery timing decisions must be individualized based on the complete clinical picture. If any condition listed above is present, consult your provider — do not rely on this tool.
Start with the essential information. Today's date is auto-filled.
Each prior birth modestly shifts labor earlier. The largest change occurs between the 1st and 2nd pregnancy. Grand multiparity (≥5 births) carries additional risks not captured here.
A uterine scar does not prevent spontaneous labor onset, but has critical implications for delivery planning. Uterine rupture risk with spontaneous labor is ~5.2 per 1,000 (vs 1.6/1,000 with elective repeat cesarean). This tool estimates timing only — it does not assess VBAC candidacy.
These factors are backed by published evidence and will shift your probability curve.
Cervical dilation and effacement at a prenatal visit after 37 weeks are the strongest near-term predictors of spontaneous labor onset. Each centimeter of dilation carries a 50% increase in the hazard of spontaneous labor (aHR 1.5/cm); women >1 cm dilated are 3× more likely to labor spontaneously within the week. Effacement adds independent predictive value. For labor progress modeling using dilation, station, effacement and contraction data, see Hamilton, Romero et al. AJOG 2024. · Rosenbloom et al. AJOG 2019 · Hamilton, Romero et al. AJOG 2024
Personalized day-by-day probability based on your inputs.
| Gestational Age | Weeks From Today | Daily Prob. | Still Pregnant |
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Publications and data sources used to build this calculator.
This tool was created by Professor Amos Grünebaum, MD — Professor of Obstetrics and Gynecology. It is published on ObGyn Intelligence (obmd.com) and LiveEvidence.com.
The calculator answers a simple but important question: What is the probability of going into spontaneous labor on any given day — starting from today? It generates a personalized, day-by-day probability chart by combining a population-level survival curve with evidence-based adjustments for your specific clinical profile. The result is conditional: probabilities reflect the chance of labor given that you have not yet delivered, which is statistically and clinically the correct framing.
The baseline timing distribution is calibrated to U.S. and Scandinavian registry data for low-risk singleton pregnancies, with a median near 40 weeks and a realistic right tail extending to 42 weeks. A skewed distribution is used rather than a symmetric normal curve — because the distribution of spontaneous labor is not symmetric: the mode (most common day) falls after the median, and approximately 10% of singleton births occur before 37 weeks, consistent with CDC natality data.
Each clinical factor you enter — BMI, parity, prior preterm or post-term history, cervical length, cervical exam findings, twins, age — applies a day-shift derived from published odds ratios and hazard ratios in peer-reviewed literature. These shifts move the probability curve earlier or later. The chart then displays: (1) the daily probability of labor onset on each specific day, and (2) the cumulative probability of still being pregnant — both starting from today.
Most labor probability tools use a symmetric normal distribution that predicts fewer than 3% of births before 37 weeks — far below the observed 9–10%. This calculator uses a skewed distribution that correctly reflects real-world preterm birth rates. It also incorporates clinical risk factors derived from published evidence, not just gestational age. Cervical length, dilation, effacement, parity, BMI, and obstetric history all shift the curve based on specific published hazard ratios — not general assumptions.