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 a skewed normal distribution.
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 or 13 days. 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,  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. 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.
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. 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 and thinking ahead? You might enjoy our other pregnancy apps, including a Week by Week Pregnancy Calander. 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 emphizing 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.
Wanting to become pregnant? Our Time to Conception Calculator can estimate how long it may take.
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