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:

Conclusion

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.

Results:

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.  

Method:

• 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.