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Change graph y-axis to probability of labor:
What does this mean for you and your pregnancy? The Labor Probability Calculator can calculate your odds of labor given how far along you are.

About the Labor Probability Graph

The Labor Probability Graph was designed for anyone who enjoyed the Labor Probability Calculator and wanted a little more detail about the underlying distribution. The Graph uses a left skewed normal distribution to modal the odds of spontaneous labor.

About the Model To derive our model, we started with Naegele's rule. Naegele's rule estimates a woman's due date as 280 days (40 weeks) from her last menstral period. It's the standard your OB/Gyn probably used when calculating your due date, unless ovulation was known or a dating ultrasound was preformed.

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]. Normal distributions are 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.

The CDC has found that 9.6% of babies are born prematurely, before 37 weeks. The normal distribution with mean 281 and standard deviation of 9 or 13 would predict less than 3% of babies are born prematurely (.03% and 2.7% respectively). In fact, [1] specifically excluded preterm deliveraries from their analysis which is why their model predicts so few preterm babies.

Another drawback is that the normal distribution is a symetric distribution. That means two points equally far from the mean will have the same probability. A symetric distribution that predicts 10% of babies will be born before 37 weeks will also predict 10% of babies will be born after 43 weeks. In a symetric 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, convential 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 sponeous labors occuring prematurely, and predicted roughly half of all women would go into labor before their due date and half after.

  1. [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. [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 Labor Probability Graph and the Labor Probability Calculator You may notice the probabilities differ between the Labor Probability Graph and the Labor Probability Calculator. That is because the two apps are modeling two similar sounding, yet different events. This Labor Probability Graph 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. Afterall, 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).

This website is intended for informational & entertainment purposes only. This website is not intended to be considered medical advice.


Pregnant? You may enjoy our other pregnancy apps like the personalized week by week calander.

Into Probability Distributions? You may also be interested in our Daily Miscarriage Probability Chart, which gives both the probability of miscarriage and probability of carrying until birth.

Wanting to become pregnant? Our Time to Conception Calculator can estimate how long it may take.

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