Enter Your Details to Get a Spontaneous Labor Timing Estimate

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This model is intended for a current singleton pregnancy with no scheduled induction or planned cesarean.

Are you currently pregnant with one baby?

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Do you currently have a scheduled induction or planned cesarean?

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Has your maternity care team told you that a complication may affect when delivery should happen?

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Are you currently having possible labor or urgent pregnancy symptoms?

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Was this pregnancy conceived through IVF, including a procedure called ICSI? ℹ️

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Have you previously given birth at 20 weeks or later, regardless of outcome? ℹ️

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How many miscarriages have you had? ℹ️

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Maternal Age: ℹ️

Pre-pregnancy BMI: ℹ️

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Smoking during pregnancy: ℹ️

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How the Calculator Works

General Advanced

Plain-language methodology

1. What this estimates

This calculator estimates when labor may begin on its own, including contractions starting or the water breaking before labor. It does not predict a scheduled induction or cesarean, an emergency cesarean, or the final type of delivery.

The forecast starts with gestational timing patterns reported for singleton pregnancies, then uses the answers in the calculator to make modest adjustments. Most large studies record gestational age at birth rather than the exact time labor began, so those birth-timing patterns are used as a practical starting point and checked against a late-term study of spontaneous labor onset.

2. How the forecast is built

The model combines two parts: spontaneous onset before 37 weeks and spontaneous onset from 37 weeks onward. This lets the earlier part of the forecast respond to factors linked with spontaneous preterm birth without forcing the main due-date curve into an unrealistic shape.

What you tell us	How it affects the forecast
Previous birth	Moves likely dates about two days earlier than for someone having their first baby.
Previous spontaneous birth	Moves likely dates a little earlier or later depending on when that labor began.
Previous preterm births	Increase the chance that labor starts before 37 weeks. Earlier or multiple preterm births increase it more.
IVF or ICSI	Modestly increases the chance that labor starts before 37 weeks.
Smoking, age, BMI, and miscarriage history	May move likely dates slightly earlier or later, or change the chance that labor starts before 37 weeks.

3. Population anchors

The baseline before-37-weeks share is 5%, rounded from a large U.S. study that reported a 4.5% spontaneous preterm delivery rate among singleton births. The term curve is anchored to reported median timing of about 284 days for first births and 282 days for later births. Its late-term tail is also checked against a large cohort of uncomplicated pregnancies still ongoing at 41 weeks.

4. Updating as pregnancy progresses

Each calculation uses how far along you are today. Dates that have already passed are removed and the remaining probabilities are re-scaled. The main result cards, weekly chances, and graph therefore describe the remaining forecast from today forward.

5. What you will see

Most-probable window - the shortest date range containing about half of the remaining modeled chance.
Most probable date - the single date with the highest modeled likelihood.
Chance-by dates - dates by which the remaining forecast reaches 10%, 50%, and 90%.
Chance before 37 weeks - the part of the remaining forecast that falls before 37 weeks.

Each adjustment is deliberately conservative. When several factors apply together, the combined estimate is a practical model assumption rather than a directly studied risk profile. Published studies consistently show that exact labor timing remains difficult to predict for one person.

References

Smith GC. Use of time to event analysis to estimate the normal duration of human pregnancy. Human Reproduction. 2001;16:1497-1500 (PubMed)
Gyamfi-Bannerman C, Ananth CV. Trends in spontaneous and indicated preterm delivery among singleton gestations in the United States, 2005-2012. Obstetrics & Gynecology. 2014;124:1069-1074 (PubMed)
Goldenberg RL, Culhane JF, Iams JD, Romero R. Epidemiology and causes of preterm birth. The Lancet. 2008;371:75-84 (PMC)
Marquette GP, Hutcheon JA, Lee L. Predicting the spontaneous onset of labour in post-date pregnancies. Journal of Obstetrics and Gynaecology Canada. 2014;36:391-399 (PubMed)
Ananth CV, Getahun D, Peltier MR, et al. Recurrence of spontaneous versus medically indicated preterm birth. American Journal of Obstetrics and Gynecology. 2006;195:643-650 (PubMed)
Ng KYB, Steer PJ. Prediction of an estimated delivery date should take into account both the length of a previous pregnancy and the interpregnancy interval. European Journal of Obstetrics & Gynecology and Reproductive Biology. 2016;201:101-107 (PubMed)
Shah NR, Bracken MB. Maternal cigarette smoking and preterm delivery: a systematic review and meta-analysis. American Journal of Obstetrics and Gynecology. 2000;182:465-472 (PubMed)
Oliver-Williams C, Fleming M, Wood AM, Smith G. Previous miscarriage and the subsequent risk of preterm birth in Scotland, 1980-2008. BJOG. 2015;122:1525-1534 (PubMed)
Han Z, Mulla S, Beyene J, et al. Maternal underweight and the risk of preterm birth and low birth weight. International Journal of Epidemiology. 2011;40:65-101 (PubMed)
Oberg AS, Frisell T, Svensson AC, Iliadou AN. Maternal and fetal genetic contributions to postterm birth. American Journal of Epidemiology. 2013;177:531-537 (PubMed)
Mercer BM, Goldenberg RL, Moawad AH, et al. The preterm prediction study: effect of gestational age and cause of preterm birth on subsequent obstetric outcome. American Journal of Obstetrics and Gynecology. 1999;181:1216-1221 (PubMed)
Yamashita M, Hayashi S, Endo M, et al. Incidence and risk factors for recurrent spontaneous preterm birth. Journal of Obstetrics and Gynaecology Research. 2015;41:1708-1714 (PubMed)
Cavoretto P, Candiani M, Giorgione V, et al. Risk of spontaneous preterm birth in singleton pregnancies conceived after IVF/ICSI treatment. Ultrasound in Obstetrics & Gynecology. 2018;51:43-53 (PubMed)

1. Modeled endpoint

The endpoint is spontaneous onset: regular contractions beginning on their own or prelabor rupture of membranes, before a planned induction or prelabor cesarean occurs. Gestational age is represented as days from the last menstrual period. The model is intended for current singleton pregnancies without a scheduled delivery or a known complication expected to determine delivery timing.

2. Two-part timing distribution

The unconditional cumulative distribution combines a preterm component and a term component:

F(t | X) = pi(X) x Fpre(t | X), when t < 259
F(t | X) = pi(X) + [1 - pi(X)] x Fterm(t | X), when t >= 259

The preterm component covers 20w0d through 36w6d. The term component is a truncated normal distribution from 37w0d through day 303. Its standard deviation is 9.44 days. Parity-specific locations were selected so the complete baseline distributions have medians of 284 days for people with no previous birth and 282 days for people who have previously given birth, matching Smith's reported medians.

3. Before-37-weeks component

The baseline probability pi0 = 0.05 is a rounded proxy from the 4.5% U.S. singleton spontaneous preterm delivery rate reported for 2012 by Gyamfi-Bannerman and Ananth. That source covered live births from 24-36 weeks; the model extends a small portion of the early distribution back to 20 weeks. The broad 5%, 15%, 20%, and 60% band shares are approximate shape choices informed by Goldenberg et al.

User factors modify the baseline odds:

odds(pi(X)) = odds(0.05) x product of applicable odds ratios

Input	Implemented adjustment	Evidence interpretation
Smoking during pregnancy	OR 1.3	Rounded from the 1.27 pooled estimate in Shah and Bracken.
Age <20 / 35-39 / 40+	OR 1.2 / 1.05 / 1.1	Conservative age-group adjustments; 20-34 is the reference.
BMI <18.5	OR 1.3	Conservative translation of the underweight association in Han et al.; other BMI groups use OR 1.0 for this component.
One / two or more miscarriages	OR 1.1 / 1.4	Conservative dose-sensitive translation of Oliver-Williams et al.
Prior spontaneous event at 20-22w6d	No additional adjustment	Recorded as history, but the available studies do not provide a stable coefficient for this group.
Prior spontaneous birth at 23-34w6d / 35-36w6d	OR 3.0 / 2.0	Conservative values below the 3.6 adjusted recurrence estimate reported by Ananth et al.
Two or more prior spontaneous preterm births	Additional OR 1.5	Applied when the earliest was at 23 weeks or later; a conservative translation of the 2.26 adjusted recurrence estimate reported by Yamashita et al.
Prior birth before 35w following induction or prelabor cesarean	OR 1.4	Conservative value below Ananth et al.'s 1.6 estimate for later spontaneous preterm birth; not multiplied when a stronger spontaneous-preterm adjustment already applies.
IVF or ICSI	OR 1.5	Conservative value below the 1.63 age-and-parity-matched estimate in Cavoretto et al.

The resulting value is bounded to 2-35%. If a previous spontaneous birth happened from 23 to 36 weeks, the preterm bands receive a small additional weighting near the earliest reported gestational age because recurrence tends to cluster near prior timing. IVF/ICSI and a previous birth following induction or prelabor cesarean affect only this component, not the term-date curve.

4. Term component

The term distribution is adjusted by exponential density tilting:

fterm(t | X) is proportional to f0(t) x exp(k(X) x (t - 280))

Age 35-39 and 40+ use k = 0.004 and 0.008. BMI 25-29.9 and 30+ use k = 0.004 and 0.012. These deliberately modest shifts extend the direction reported among pregnancies still ongoing at 41 weeks across the term period.

When a previous birth began spontaneously, the reported birth day contributes 0.0015 x (prior day - 280) to the tilt, bounded from -0.025 to 0.035. The form uses the earliest spontaneous preterm birth when one occurred and otherwise the most recent spontaneous term birth. This preserves the reported tendency for gestational length at birth to recur while applying strong regression toward the population mean. The term baseline also receives a broad cohort-level tail check against the 67.6% chance of spontaneous onset from 41w0d through 41w6d reported by Marquette et al.; that percentage is not a parity-specific estimate.

5. Conditioning on today

If the pregnancy is at day g and spontaneous onset has not occurred, every displayed result uses the remaining distribution:

Fremaining(t | X, g) = [F(t | X) - F(g | X)] / [1 - F(g | X)]
fremaining(t | X, g) = f(t | X) / [1 - F(g | X)], when t >= g

The most-probable window is the shortest interval containing 50% of this remaining probability. Near-term chances and the 10%, 50%, and 90% chance-by dates are calculated from the same conditional distribution.

6. Interpretation

This is a literature-derived population estimate built from published summary statistics, not a clinically validated individual prediction model. The coefficients are conservative translations chosen for a simple consumer questionnaire. They do not represent interactions, clinical measurements, scheduled delivery, or every factor that may affect labor timing.

References

The peer-reviewed sources and links are listed in the General methodology tab.

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Frequently Asked Questions

How accurate is this labor calculator?

It provides probabilities, not certainties. Estimates are based on large studies and the information you enter. Always consult your healthcare provider.

Will you store my information?

Free calculator entries stay in your browser and are saved only if you choose "Save for 30 days." If you purchase and sign in to the Labor-Ready Plan, an encrypted copy of the normalized forecast inputs and your planning details is synchronized so your plan can open in another signed-in browser. Raw height and weight are not stored. Analytics receives no calculator answers, dates, probabilities, or plan content. Read the privacy and health-data policy.

What factors affect my estimate?

Previous birth history, miscarriage history, smoking, BMI, and maternal age can shift the modeled timing curve.

Time Frame	Probability
Next 24 hours
Next 3 days
Next 7 days
Next 14 days

Enter Your Details to Get a Spontaneous Labor Timing Estimate

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Chance Before 37 Weeks

How the Calculator Works

Plain-language methodology

1. What this estimates

2. How the forecast is built

3. Population anchors

4. Updating as pregnancy progresses

5. What you will see

References

1. Modeled endpoint

2. Two-part timing distribution

3. Before-37-weeks component

4. Term component

5. Conditioning on today

6. Interpretation

References

Thinking about baby names?

Frequently Asked Questions

How accurate is this labor calculator?

Will you store my information?

What factors affect my estimate?

Notes