OBGYN INTELLIGENCE

When Might Labor Start?

Enter your due date and pick a date. See your chance of going into labor on your own, building up day by day, through two weeks past your due date.

Your due date The estimated date you were given (40 weeks).

Looking from this date Today by default. The chances build forward from here.

What this shows. These are population estimates for spontaneous labor — labor that starts on its own — if a pregnancy is simply allowed to continue. They do not predict a scheduled induction or a planned cesarean. Every pregnancy is different. This tool does not replace your own doctor or midwife.

A note on timing. Most providers discuss induction around 41 weeks and recommend it by 42 weeks, because risks rise as a pregnancy goes past the due date. Use these numbers for understanding, not for decisions — those belong with your care team.

View the evidence & sources →

Evidence & Sources

The numbers in this tool come from published studies of when spontaneous labor begins. Daily values between published points are smoothed estimates.

How the curve is built

The tool models the cumulative chance that spontaneous labor has begun by a given gestational age, then shows it as a chance counting forward from the date you pick (assuming you have not yet delivered on that date). The shape is anchored to published data near and beyond the due date; values in earlier weeks are smoothed estimates. It reflects expectant management and excludes planned induction or cesarean.

Key anchor points used: about 37% of spontaneous labors occur during the 40th week, roughly 80% have occurred by 41 0/7 weeks, and the median (half) falls near 40 3/7 weeks. First-time (nulliparous) pregnancies tend to deliver slightly later than pregnancies after a prior birth.

Before 37 weeks: the chance is small but never zero. About 1 in 10 US births are preterm (before 37 weeks), and spontaneous preterm labor is part of that. The tool shows a low, gradually rising chance across the preterm weeks rather than a flat 0%. These early-week values are smoothed estimates.

References (Vancouver)

1. Corbett GA, Dicker P, Daly S. Onset and outcomes of spontaneous labour in low risk nulliparous women. Eur J Obstet Gynecol Reprod Biol. 2022;274:142-147.Verified doi:10.1016/j.ejogrb.2022.05.010. Retrospective cohort of 12,323 low-risk first-time mothers with spontaneous labour. Median gestation at onset 40.1 weeks; ~37% of labours in the 40th week; 80.5% by 41 0/7 weeks. Primary source for the curve shape at and beyond the due date.

2. Smith GC. Use of time to event analysis to estimate the normal duration of human pregnancy. Hum Reprod. 2001;16(7):1497-1500.Verified doi:10.1093/humrep/16.7.1497; PMID 11425837. Kaplan-Meier analysis of 1,514 pregnancies with ultrasound-confirmed dating. Median time to spontaneous (non-elective) delivery was 283 days (40 3/7 weeks). Corroborates the 50% point.

A population-based post-dates registry study (spontaneous labour within the week after 41 0/7 in roughly two-thirds of those still pregnant) supports the curve's tail; its full citation can be added after a reference field-check.