Move your cursor over the graph to see the associated probabilities for each date. Click to lock/unlock your cursor in place.Tap the graph to see the associated probabilities for each date.

About the Labor Probability Calculator

Will you go into labor today? Tomorrow? Next week? The labor predictor estimates the odds of spontaneous labor on a given day based on where you are relative to your due date using statistical modeling.

Important Tool Caveat

This tool predicts the odds of spontaneous labor without consideration of possible negative outcomes. It should not be used when making medical decisions.

About the Model

Prior research has shown that the distribution of spontaneous labor approximates a normal distribution with a standard deviation of 9 [1] or 13 days [2]. These studies provide the initial building blocks for our model, but fail to paint a complete picture. Neither distribution does a good job of estimating the number of preterm deliveries. The CDC has found that 9.6% of babies are born prematurely, before 37 weeks. The normal distribution based on [1] and [2], however, would predict less than 3% of babies are born prematurely (.03% and 2.7% respectively). In fact, [1] specifically excluded preterm deliveries from their analysis which is why their model predicts so few preterm babies, which explains why it's model predicts so few of them.

The normal distribution is favored in these types of applications for it's simplicity and tendency to fit the data. In this case, however, the normal distribution is likely an over simplification. A key drawback is that the normal distribution is a symmetric distribution. That means two points equally far from the mean will have the same probability. A symmetric distribution that predicts 10% of babies will be born before 37 weeks will also predict 10% of babies will be born after 43 weeks. For all the births prior to 30 weeks, there would be a similar number of births after 50 weeks. Additionally, in a symmetric distribution the median (point which half of all women would have gone into labor), is equal to the mode (most common day to go into labor.) In prior studies, however, the mode date is typically after the median [2]. Although not scientific, conventional wisdom is that the most common day to go into labor is around 41 weeks.

Taken together, these data points suggests that a skewed normal distribution might be more appropriate. The skewed normal distribution is a family of distributions that includes the normal distribution, however the skewed normal distribution need not be symmetric. Where the normal distribution is defined by two parameters (mean and standard deviation), the skewed normal distribution is defined by three (location, shape and scale). Using mean squared error we identified a skewed normal distribution that closely approximates the normal distribution identified with prior research (MSE of 0.002), accounted for 10% of spontaneous labors occurring prematurely, and predicted roughly half of all women would go into labor before their due date and half after.

[1] H. Kieler; O. Axelsson; S. Nilsson; U. Waldenströ (1995). "The length of human pregnancy as calculated by ultrasonographic measurement of the fetal biparietal diameter". Ultrasound in Obstetrics & Gynecology. 6 (5): 353–357. doi:10.1046/j.1469-0705.1995.06050353.x

[2] Bergsjø P, Denman DW 3rd, Hoffman HJ, Meirik O. (1990). "Duration of human singleton pregnancy. A population-based study.". Acta Obstet Gynecol Scand: 197–207.

Differences between Datayze's model and those found on other websites

The key difference between our model and every other model out there (that we know of) is our model better incorporates the possibility of preterm spontaneous labor. Like [1], most online models predict the probability of spontaneous labor before 37 weeks is approximately zero. As you can see from the Full Chart we estimate the probability of spontaneous labor before 37 weeks as 9.6%, matching the number from the CDC. If you're past 37 weeks, this distinction will be less important. Before 37 weeks and our model will serve you much better.

Differences between Labor Probability Chart and the Labor Probability Calculator

You may notice the probabilities differ between the Labor Probability Chart and the Labor Probability Calculator. That is because the two apps are modeling two similar sounding, yet different events. This Labor Probability Chart shows the probability of spontaneous labor for a pregnant woman without considering how far along she is in her pregnancy. The Labor Probability Calculator shows the probability of spontaneous based on how far along she is by renormalizing the distribution to include only the possible remaining days in a woman's pregnancy. After all, for a woman who hasn't gone into labor by today the probability of spontaneous labor starting yesterday is, by definition, 0%. Statistically speaking it's the difference between the probability of labor at 40 weeks 0 days in general, p _labor (40w0d), and the probability of labor at 40 weeks 0 days for a woman who is already 39 weeks along, p _labor (40w0d|39WeeksAlong).

Take the Probabilities With You

Wondering what the probability is of going into labor before your due date, or before your induction date? Want to bookmark the Labor Probability Calculator with all it's data so you don't have to keep re-entering your date info it day after day? If you check the "Remember This" check box, a cookie will be created that stores your due date on your computer. Alternatively, you can save the parameterized URL by prepending the string ?duedate=MM/DD/YYYY to the end of the URL where MM/DD/YYYY corresponds to your due date. For example, the parameterized URL for the above input with a due date would be:



This website is intended for general information & entertainment purposes only. This website is not intended to be considered medical advice. I am happy to answer questions about the apps, and the mathematics behind them. I cannot answer medical questions.

Pregnant and thinking ahead? You might enjoy our other pregnancy apps, including a Week by Week Pregnancy Calendar. If it's really early in your pregnancy you may be interested in our Miscarriage Odds Reassurer, which is designed to help ease any miscarriage fears by emphasizing the odds of carrying to birth rather than the odds of miscarriage, or our Daily Miscarriage Probability Chart which shows how the probability changes over the course of the first twenty weeks.

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