Come on out!

Hurray! You’re pregnant, you’ve passed the first trimester and you’re ready to tell the world! But when exactly is your bundle of joy going to make his debut? Everyone will tell you forty weeks, but will that be accurate? At Babylytics, we looked at 20 factors to help see what has the largest impact on due date.   No one had even mentioned the idea that the baby could come LATE!

To kick it off, we’re going to spoil the article and say 40 weeks is  NOT accurate.  The average gestation period (time since conception) of our 2.7 million babies is 38.7 weeks, with a standard deviation of 1.9 weeks.  This means that 95% of all babies are born between 34.9 and 42.5 weeks! (No big shocker there.)  So what helps determine where in that spectrum you fall?


                         Sample: 1,261,509, Average: 38.02, Standard Deviation: 2.5

But what about those that had a C-Section? Does that sway in any particular direction?

                       Sample: 1,261,509, Average: 38.02, Standard Deviation: 2.5

Wow! Those with a C-section are miraculously quite punctual coming out at the 39 weeks! Over 40% of all C-Section babies come out on that week, with the rest scattered about.  From the tail of our graph, we can see that there is a greater share than those that came out naturally in the earlier weeks.  This includes all those that had an emergency C-section.  Our standard deviation also increased by .6 weeks because of this tail.  For the remainder of this article, we will be excluding all C-Sections that were at 38 weeks or afterward because they are either planned or were an emergency after the baby had come to term.

Previous Preterm Pregnancies

Next, we look at your history to see how it may affect this baby.  We are going to start with if you’ve ever had a previous baby who was preterm.  The Mayo Clinic defines preterm labor as entering labor between 20 and 37 weeks of pregnancy. It seems a reasonable hypothesis that those who’ve had a previous early baby would probably be more likely to have another.  So let’s check to see if that hypothesis is correct.

We see our assumption holds up!  Those who had one or more previous pre-term babies averaged a gestation period of 37 weeks, as compared to those who never had a preterm baby at 38.5 weeks (roughly our average of the total population!).   With an average of 37 weeks, it is unfortunately the case that many of these babies will be pre-term again. 


If you’re put in the uncomfortable position of my wife, our boy JR stayed put for 42 weeks! So, what made him not want to come out? Well his momma is a rather smart cookie, so I looked into education as a factor.

This is the most interesting chart I found in the dataset.  Those who have less than a high school education tended to have the longest natural gestation periods, and those with a doctorate are slightly less than those with a Master’s.  My hypothesis is those with less than a high school education are just those that are very young (under 16) and have the highest fertility rates…Boy was I wrong!

I added the average ages at the time of pregnancy of each group above their education levels, it turns out those with less than a High School education are the same age as those with an associate’s degree at time of birth.  And if you look at their gestation period, they’re the same.

As for rest, the older the mom, the higher the degree, the longer the gestation period. I’m not entirely sure why that is the case but this tends to go against the conventional wisdom.   My best guess is that with higher education comes higher income which leads to better nutrition and more frequent doctor visits.

When looking at the Father’s Education, the impact is identical.

Full disclosure: These differences are very minor with the largest being between HS no diploma and those with a Master’s degree, which is still less than half a week.  It’s just fun to think about!

Other factors

What about number of previous children? Age? Gender of the baby? Unfortunately, none of these have any large impact.  What does have the biggest impact are ‘abnormal conditions’, meaning if the baby comes out on time, he has a much higher likelihood of being healthy.  This is less of a predictor and more of a reason to make sure you follow the doctor’s advice.

So what did we learn?

Not much. If you had one preterm birth, you are more likely to have another.  The higher your education level, the longer the baby stays in momma’s belly. But these are relatively negligible differences.  If you want to see how your pregnancy might turn out, head over to our Pregnancy Predictor dashboard and look at our 4 million other moms and how you compare.


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