Wednesday, March 2, 2011

Does a Baby's Birth Weight Correspond With A Mother's Weight Gain?

This is part one on a series of pregnancy and birth outcomes.

I've been messing around with the birth certificate data, but hadn't formally prepared anything to be posted because honestly that is the least fun part.  But Lauren at Hobomama (great blog!) had a post about her midwife harping on weight and she questioned whether a mother's weight gain really influences the weight of their baby.  So I thought, "Hmmm lets see".  Here are the results:


On average, the more weight a mother gains, the more her baby will weigh.  However, the average difference in a baby's weight, between a mother who gains 25lbs versus a mother who gains 70lbs, is only 1/2 lb.  There seems to be more involved in fetal macrosomia than a mother's weight gain.  If you feel comfortable that you eating nutritiously and are being as physically active as you can, then you are probably fine. If a provider tells you to eat less to avoid having a huge baby, this statement is based on exaggerated evidence at best.


Click to make larger

  • You can see that in general as a mother gains weight her baby on average is larger. The exception is when weight loss occurs or only 0-9lbs of weight gain, however the confidence intervals overlap in this range so it is not statistically significant.
  • The results are significant till you get to fifty pounds and over, or when the mother lost weight then the results become less relevant.  
  • Babies born from women who gain the recommended 20-29lbs weight on average 7.51bs (7lbs 8oz).  
  • Babies born from women who gain 30-39lbs weigh on average 7.66lbs (7lbs 10oz).  The 2oz difference is statistically significant.  
  • Babies born from women who gain 40-49lbs weighed on average 7.82lbs (7lbs 13oz).  The 3oz difference is statistically significant.
Just for curiosity sake here is the entire chart in a mostly full range of weight gain and loss.

  • It seems that baby weights rise when women lose a lot of weight, but the confidence intervals for this are large and thus the information isn't really meaningful.  

  • I summarized the data of all women from 2003 - 2007 who carried their babies full term (37 weeks), whose starting and delivery weights were provided and seemed reasonable (above 90lbs).  I did not use the weight gain field calculated by the WSDH because it did not include mother's who lost weight.
  • I excluded data when the weight gain was above 79lbs or whose weight loss was below 9lbs because the results were not statistically significant. 
  • I grouped all women in 10lbs ranges so 0-9, 10-19, 20-29, etc and calculated the average weight of their babies in grams, and converted it to pounds by multiplying the weight by .0022.
  • I used sql server's stdev function to calculate the standard deviation for each group.  To calculate the confidence interval I used the instructions from here and summarized as:
    • CI is the confidence interval
    • avg - is the average baby weight
    • SD - standard deviation
    • N - number of mothers in the sample
  • I graphed the results in Excel.  I included both the confidence intervals and the actual results.  To determine statistical significance I use the simplistic logic that if the confidence intervals overlap then the results are not statistically significant, if they don't overlap, then they are.  This logic is per the recommendations from the Washington State Department of Health.

Future Posts
  • Rates of fetal macrosomia compared to mother's weight gain or loss
  • Rates of fetal macrosomia compared to mother's initial weight or bmi
  • Fetal weight and fetal macrosomia compared to whether a mother has gestational diabetes or not.
These posts take a long time to write up, so I'll get to them when I can.  Please leave me a comment if you find this information useful or interesting so I know if it is worth my time and effort to write up my analysis. 


    Lauren @ Hobo Mama said...

    This is so interesting! Thank you for taking the time to put it together. (I'm much impressed!) It's intriguing to see that maternal weight gain has an effect, but such a relatively small one across the population.

    It's funny, for instance, that my 33-pound weight gain resulted in an 11 lb/13 oz baby, and that that contributes to the average 7.66 lbs but is obviously on the outer rim. And my mom gained 5 lbs when she gave birth to 9 lb/15 oz me, which was kind of random, but then gained more weight giving birth to my average-weight brothers. I realize these are only two anecdotal examples, but I appreciate your conclusion that there has to be more to macrosomia than just maternal weight gain.

    I will be very interested to see your future analyses as you have time for them! Thanks so much for writing this up.

    Katie said...

    Very interesting!

    Kristen @ Adventures in Mommyhood said...

    Very interesting! I gained 36lbs with my son and people always commented how I was going to have such a "huge baby" (I carried right out in front as well). I wanted to shout out "I told you so!" when he was 6lb 13oz, just about the same size as his sister with whom I gained less weight as I was on a restricted diet.

    I'm also quite a nerd, so I really like the statistical analysis and graphs. Looking forward to seeing more!

    Rebecca said...

    Good post! Found it interesting! I only had gained 12lbs during my pregnancy and gave birth (41weeks and some days) to a 9lb 11oz baby... who also was 23 inches long. I had been overweight at the start of the pregnancy and didn't gain anything until about 5-6 months in. I didn't even have gest. diabetes, my test results came back normal. But I'm one of those rare cases I guess, my sisters gained weight in their pregnancy's and they had 6-8lbs babies.

    Lindsey said...

    This is fascinating! I gained just about thirty pounds (carrying right out in front on my 5'1" frame) and my baby was 9 pounds.

    Jenn said...

    definitely an interesting post - thank-you!
    I am an average sized woman (probably about 5-10lbs overweight when not pregnant) and I gave birth to a 10lb 11oz, 24inch baby - no GDD either.. I gained about 40lbs during that pregnancy. I am 29weeks pregnant now, and very interested to read any information on macrosomia, and some of the causes/correlations.

    Amy @ Anktangle said...

    Wow, thanks so much for taking the time to do the statistical analysis! It was very interesting to see these graphs. I'll look forward to the next one. =)

    Momma Jorje said...

    SO awesome of you to do this for Lauren (even if she only put a rush on you doing it). I'm very keen to see the Gestational Diabetes analysis! I'm subscribing and hope I don't miss it among my other blogs.

    Betsy B. Honest said...

    I'd always wondered about that but didn't have the skills to figure it out myself. Good on ya!

    Not your average mama blogger, eh?

    FSUHSY said...

    Thanks for this information. My midwife is making this pregnancy so difficult by harping on how my weight gain is creating an enormous baby (I'm 33 weeks and I'm up to 26 lbs). What she fails to recognize (despite my pleas) is that my husband comes from a long line of huge babies. Her pestering is making me more anxious and nervous than I need to be. I really appreciate you synthesizing this info and making me feel better :)