Labor Probability Calculator
See the statistical probability of going into labor on any given day. Probabilities update in real-time as your pregnancy progresses.
This calculator uses a skewed normal distribution calibrated to epidemiological data on spontaneous labor onset. The key feature is conditional probability: as each day passes without labor starting, we remove that day from the distribution and recalculate. This means your daily probability of labor increases as your pregnancy progresses - which matches what doctors observe clinically.
What percentage of babies are born on their due date?
Only about 4-5% of babies are born on their exact due date. Most babies arrive within 1-2 weeks of the estimated due date. The due date is the median - roughly half of babies come before it and half after.
Does being a first-time mother affect when labor starts?
Yes. First-time mothers tend to deliver about 2 days later on average than mothers who have had previous pregnancies. This is based on Smith (2001) and other peer-reviewed studies analyzing thousands of births.
What does conditional probability mean in this context?
Conditional probability means "given what we already know." If you are 40 weeks pregnant today, we know labor has not started yet. So we remove all the days before today from the probability distribution and recalculate. This makes the probability of labor on any remaining day higher than the original unconditional probability.
How accurate is this calculator?
This calculator is based on population-level statistics from peer-reviewed research (Smith 2001, Jukic 2013, Kieler 1995). Individual pregnancies vary significantly - natural variation spans about 5 weeks even among healthy pregnancies. The calculator provides a useful statistical picture but cannot predict exactly when your labor will begin.