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Are you currently pregnant with one baby?

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Do you currently have a scheduled induction or planned cesarean?

No Yes

Has a pregnancy complication been identified that may affect delivery timing?

No Yes or unsure

Are you currently having possible labor or urgent pregnancy symptoms?

No Yes or unsure

How would you like to enter your dates? ℹ️

Last period Established due date

Enter one date. We'll calculate the other.

Established Due Date: ℹ️

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

No Yes Unsure

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

None One Two or more

Maternal Age: ℹ️

Pre-pregnancy BMI: ℹ️

I know my BMI Enter height & weight

BMI Range:

Smoking during pregnancy: ℹ️

No Yes

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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. Scheduled deliveries are outside the forecast. It also does not predict an emergency cesarean or how the baby will be delivered.

The forecast starts with gestational timing patterns reported for singleton pregnancies, then uses the answers in the calculator to make modest adjustments. An entered due date is converted to an inferred last menstrual period by subtracting 280 days; an entered last menstrual period is used directly. 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	For a reported birth at 23 weeks or later, moves likely dates a little earlier or later depending on when that labor began.
Previous spontaneous preterm births	For a reported birth at 23 weeks or later, increase the chance that labor starts before 37 weeks. Earlier or multiple preterm births increase it more. An event at 20-22 weeks is retained but does not receive a numerical adjustment.
Previous medically recommended preterm birth	Modestly increases the before-37-weeks chance when a stronger spontaneous-preterm adjustment is not already being used.
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. The calculator can be used earlier in pregnancy, but the modeled labor distribution begins at 20 weeks. It stops accepting calculations at 42 weeks, while retaining a small modeled tail through 43 weeks + 2 days for forecasts calculated earlier.

5. What you will see

Most-probable window - an approximate shortest interval containing 50% of the remaining modeled chance. Its displayed calendar dates are rounded outward, so the displayed range contains at least 50%.
Most probable date - the date with the highest modeled density in the implemented search from 31 weeks + 3 days onward, or from today when today is later.
Chance-by dates - the first whole calendar 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.
Weekly chances - the remaining chance in each gestational week, including only the unelapsed part of the current week. Very small values below 0.1% more than five weeks ahead are omitted.

Published results support shifts of days rather than weeks, so the calculator keeps these adjustments small. It cannot identify the exact day labor will begin.

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 entered LMP is used directly; an entered due date is converted to an effective LMP as EDD - 280 days. Calendar dates are normalized with UTC-safe date arithmetic.

The model is intended for current singleton pregnancies without a scheduled delivery, a known complication expected to determine delivery timing, or current symptoms requiring care. An inferred LMP in the future and gestational age at or beyond day 294 (42w0d) are rejected. The probability distribution itself begins at day 140 (20w0d) and has finite support through day 303 (43w2d).

2. Two-part timing distribution

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

F (t | X) = {\begin{cases} π (X) \times F_{pre} (t | X) & when t < 259 \\ π (X) + [1 - π (X)] \times F_{term} (t | X) & when t \geq 259 \end{cases}

The preterm component covers day 140 through just before day 259 (20w0d-36w6d). The term component is a normal distribution truncated from day 259 through day 303. Its standard deviation is 9.43683160171685 days. Baseline term locations are 284.89435646311017 days with no previous birth and 282.7264705050826 days with a previous birth. With the baseline 5% preterm mass, these locations give complete-distribution medians of approximately 284 and 282 days.

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 preterm component assigns 5%, 15%, 20%, and 60% of its mass to days 140-195, 196-223, 224-237, and 238-258 respectively, with uniform density inside each band. These broad shares are approximate shape choices informed by Goldenberg et al.

User factors modify the baseline odds:

odds (π (X)) = odds (0.05) \times \prod 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%. For an earliest previous spontaneous preterm birth on modeled day d from 161 through 258, each baseline band share is multiplied by 1 + 0.5 × exp(-0.5 × ((band midpoint - d) / 21)²), then all four shares are normalized to sum to one. This gives modest extra weight near the previous 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:

f_{term} (t | X) \propto f_{0} (t) \times \exp (k (X) \times (t - 280))

The implemented tilt k is the sum of applicable age, BMI, and previous-birth-timing terms. Age 35-39 and 40+ contribute 0.004 and 0.008. BMI 25-29.9 and 30+ contribute 0.004 and 0.012. The code applies the equivalent normal-density location shift location = baseline location + k × 9.43683160171685² before truncation. 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:

F_{remaining} (t | X, g) = \frac{F (t | X) - F (g | X)}{1 - F (g | X)}

f_{remaining} (t | X, g) = \frac{f (t | X)}{1 - F (g | X)}, when t \geq g

Near-term chances evaluate this conditional CDF at g + 1, g + 3, g + 7, and g + 14. The 10%, 50%, and 90% chance-by points are found by binary search and rounded forward to the first whole gestational day reaching the threshold.

The most-probable window is approximated by scanning lower quantiles from 0 through 0.5 in steps of 0.001, pairing each with the quantile 0.5 higher, and choosing the narrowest pair. The displayed start is rounded down and the displayed end is rounded up, so the calendar-day window contains at least 50%. The most probable date scans conditional density in 0.1-day steps from max(g, 220) through day 303 and rounds the selected day to the nearest calendar date.

Weekly probabilities use the remaining part of the current gestational week and each later week through day 303. Values below 0.1% are omitted only when they are more than five weeks beyond the current week.

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. The model does not use cervical examinations or ultrasound measurements, and it does not cover scheduled delivery.

References

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

Frequently Asked Questions

What does the 50% labor window mean?

The main window is the shortest span containing at least 50% of the remaining forecast. Its width changes with how far along the pregnancy is and the answers entered; it is not a guarantee that labor will begin inside the window.

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 spontaneous labor timing and preterm-birth history have the largest modeled historical effects. The remaining factors make smaller adjustments, which the result lists individually.

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

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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

Frequently Asked Questions

What does the 50% labor window mean?

Will you store my information?

What factors affect my estimate?

Notes