Wednesday, March 9, 2011

Baby Birth Weight and Mother's Pre-pregnancy Weight

This is part four  in my Pregnancy and Birth Outcomes Analysis and continues to explore fetal macrosomia.

 Pre-pregnancy Weight vs Baby's Birth Weight

It does not seem that a mother's pre-pregnancy weight is a huge factor in determining how much their baby will weigh, except in the case of women who are considered underweight, their babies do tend to be a bit smaller.  


  • There seems to be a statistically significant trend in a mother's pre-pregnancy weight and the birth weight of her baby, however the confidence intervals do start to overlap above 200lbs.  
  • The steepest increase in average baby weight is seen in the 90-140lb range after that the increase is very small (the average baby weight increases by approximately .5-1oz for every 20lb group).  

Mother's Pre-pregnancy BMI vs Baby's Birth Weight
  • This is similar to the above analysis.  Each mother was put into a group based on her pre-pregnancy BMI (Body Mass Index), which is a height to weight ratio, that some health professionals use.  I used the common groupings of: 
    • Less than 18.5 (underweight), 
    • 18.5 - 24.9 (normal), 25-29.9 (overweight). 
    • Commonly 29.9+ (obese) is all lumped together, but I decided to further categorize it and added 29.9 - 39.9 
    • 40 - 49.9 
    • 50 - 59.9 
    • Over that the sample size was too small and the confidence intervals too large.

  • A baby's birth weight is higher for women who had a higher pre-pregnancy BMI.  It is statistically significant till you get a BMI over 40.
  • The greatest difference  (5.6oz) is found among baby's born to mother's whose BMI is considered underweight compared to mother's whose BMI is considered normal.


See here to see information about the data.
  • The mother's are grouped by their pre-pregnancy weight in 20lb increments (starting at 90-110lbs, ending at 320-340lbs).  The average baby weigh (in pounds) is calculated for each group.  For the groups of mother's weighing more than 340lbs the confidence intervals were wide due to the small sample size.
  • The data is to full-term babies (at least 37 weeks)
  • The confidence intervals were calculated in the same manner as this post.

I'm going to wrap up the exploration in fetal macrosomia by comparing the rates of complications of baby's who are large versus babies who aren't.  

1 comment:

Lauren Wayne said...

This makes sense to me. I would have expected that lower-weight women would tend to have lower-weight babies. It's interesting that BMI has an effect, too. But what's even more interesting is how small of an effect either has, except with low-weight and underweight women.

Thanks again! Can't wait for Part 5. :)