Saturday, March 5, 2011

Baby Birth Weight versus Gestation

I got some positive responses so I'll keep posting my analysis of the birth data.  So this is part 2 in my Pregnancy and Birth Outcomes Analysis and further explores possible relationships between various variables and fetal macrosomia.

See here to see information about the data.  

Method:
  • Calculated the average birth weights for full term baby's born grouped by how long their gestation period was.
  • Confidence intervals were calculated in the same manner as this post.


    Results:


    Comparing Average Baby Birth Weights 
    to Their Gestation Time


    • The confidence intervals did not overlap except for babies born at 43 weeks (there were only 117).  
    • It is not statistically significant that babies born at 43 on average weighed less.  
    • The increase in difference in average weight is not linear.  
    Conclusion
    I thought these results were really interesting.  I'm not sure if you can really use this information to determine fetal growth, but knowing the uncertainty involved in estimating weight via ultra-sound I can't think of a better way to estimate fetal growth than this, so I'm going to go with it.

    I had the understanding that a baby gained about half a pound per week the last month of pregnancy.  I'm not sure where I heard this.  I read a lot, so it must have been somewhere, but it doesn't seem to be true.  The average weight gain is just less than 6oz (so just less than 1/3lb per week) per week from 36 - 40 weeks, and it isn't a linear gain, meaning it isn't the same amount of weight each week.  Their growth rate seems to slow the longer they gestate.  I was pregnant with my second child four weeks longer than with my first and as every week passed by I remember thinking that my baby just gained 1/2lb and despite trying not to be, I was a bit concerned. My second ended up being just 3/4lb heavier than my first.  This information would have been nice to know.

    It seems like there could be contradictory conclusions about this data in regards to whether inducing labor is okay.  On one hand you could say, well the baby isn't going to grow that much more anyway, so you might as well induce early (for convenience), on the other hand you could say the baby isn't going to grow that much more, so you might as well let them stay till labor happens spontaneously (to contradict the "big baby" argument).   I agree with the latter conclusion, because I believe that there is other important development that happens at the end that happens better in-utero.

    Edited To Add:  There is quite a large difference between 39 weeks and 41 weeks (11oz), so I guess there may be some basis to assume that waiting could result in a much larger baby.  But there is always the unknown of how much longer it will be before labor starts on its own... 

    * Please leave me comments if you found this interesting.

    3 comments:

    Laura said...

    Laura, this is so awesome. Thank you for sharing.

    Momma Jorje said...

    Fantastic! I am also fascinated by birth stuff, but am just so amazed and impressed by what you've done with your special skill set. :-)

    I have had a couple large-ish babies (8lb 10oz, then 8lb 9oz) and had GD with my 3rd. So the doc didn't want me to carry to 40 weeks, though I had my GD totally in check with diet. I let him strip my membranes (twice) to get labor started early. It worked and Sasha was born at 7lb 8oz... my smallest baby yet!

    Lauren @ Hobo Mama said...

    This is so cool! Sorry for taking this long to respond — I had these windows open, and my browser crashed and I forgot to get back!

    It's so interesting to hear that the weight doesn't pile on as much in the final weeks as I'd heard, too. I'm definitely in the camp of letting labor start on its own (unless truly medically advisable to induce). My first was 40 weeks, and I can't imagine he'd have been small at 38 weeks, anyhow! :)

    The article you linked to is really interesting as well. I didn't know how much important development happens right toward the end of pregnancy. It seems even more reasonable to let labor start on its own, considering not everybody has a good estimate of their due date. If you thought you were inducing someone at 38 weeks and it was really 36 weeks, that would be even more dangerous. (I think ultrasounds are supposed to be fairly good at dating, but there are always mistakes, as well as people who don't have ultrasounds, like me.)