Now if you’re anything like my wife, as you’re approaching that due date and start to feel…over encumbered, you must be wondering how big is this little guy or gal going to be?  To try and answer that, we looked at the CDC Dataset of over 4 million U.S. babies born in 2016 to figure out what the largest factors are to determine his or her size.  For this analysis I have removed those who came out before the 37-week mark for everything but the ‘gestation’ part of this article because we will be looking at these impacts 1 by 1.

Weeks in gestation

How long the bun is in the oven is the best way to know how big she’s going to be.  Babies born at the 40-week mark tend to average 8 pounds on the dot for boys, and 7.5 for girls.  (Boy values are the ones plotted, see appendix for the girl numbers above)

Babies are only 1 pound by the 21-week mark, but gain their second pound in just 5 more weeks! From there, it’s off to the races.  Starting at 32 weeks, the baby will grow 0.5lbs a week for the remainder of pregnancy (poor mommas.)  Thankfully, post 40 weeks does not have that extreme a change, but I still pity those at 42 weeks (like JR’s mom. Our ‘little’ guy came out at 9.7lbs!)


Using the same graph, what I find interesting is how the difference between boys and girls starts as early as the 21st week and continues getting wider where it is eventually 0.3lbs! (5 ounces).

Mother’s Height

Baby’s take after their parents, so mom’s height plays an important role.  While mom’s and dad’s weight could also be a factor, there are a lot more things that can alter someone’s plumpness … whereas genetics play the biggest role in your own (and your baby’s) height.

This is called a ‘box-and-whisker’ chart, where those inside the box make up the 25%-75% of all babies, and the line in the middle is the median.  The median separates exactly half of babies being born bigger than that weight and smaller than it.  (Future parents: get used to this graph, this is how a lot of standardized test results are displayed!)  The main takeaway here is that as mother’s height keeps increasing, birth weight goes with it.  Mom’s in the 54-58in bracket have babies with a median weight of around 7.1lbs, where our 68-72in moms have babies around 7.6lbs!


BMI is interesting because of how little it plays after a mom hits the 21 mark, which is what the NIH calls a ‘healthy’ range.  The NIH categorizes BMI as

When a mom’s BMI is below 18 there is a sharp drop off in birth weight, but after that there is only a slight increase in birth weight.  Now I’m not a doctor, but I can read a chart, so in my humble opinion moms should be allowed an extra scoop of ice-cream…for the baby.


In both mother and father charts, there are 2 clear splits:

  1. AIAN, White and NHOPI
  2. Black and Asian

Asian and Black babies average about 7.1 pounds for those born after 37 weeks of gestation, while NHOPI, White, and AIAN around closer to 7.5. While we do not have enough data to make an inference about cause, the large differences in size make it an important factor.  A future version of The Pregnancy Predictor will have to be updated to include it.)


Smoking while pregnant does have a clear impact on baby’s weight.  There was a significant decrease in weight, almost half a pound, when mothers smoked through their pregnancy and still a 0.3lb decrease for those who smoked beforehand.  When mothers smoked before pregnancy, they had a 75% chance of smoking during the second trimester, and a 68% chance in the third.  For those who were still smoking in the second, they had an 89% chance to keep smoking in the third!  Having a baby is tough work, but to try and have the biggest little one you can the end result is clear: try not to smoke while pregnant!

Putting it together

In the end, we have 6 big factors: Mother’s Height, Race, and Number of Weeks she’s pregnant, along with BMI, Smoking Prevalence, and Gender.  Unfortunately, there isn’t a whole lot from these that the Momma can impact.  She can kick the smokes and make sure she doesn’t skip on the dessert, but the rest is pretty pre-determined.  (Unless you believe How I Met Your Mother, then you can have lots of lemons to make a girl 😉)


Baby weight with the girl’s numbers listed above


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