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Date: | Wed, 9 Apr 2008 15:28:15 -0400 |
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Dear all:
I've already posted on the flaws of using doubling and tripling birth weights as a flawed
indicator so you could read what I've already posted in more detail about the following:
1) regression to the mean
2) catch up and catch down
3) doesn't tell you anything about recent growth history
4) weight at an older age is not as important as behavior
In addition, this whole topic has pushed me to do the math and really look at this in ways
I never did nor did anyone else when I was in nutritional sciences.
I went to the new WHO charts with breastfeeding infants and took the whole range from -
3 z-scores to + 3 z-scores. That is essentially the smallest to the largest normal healthy
babies that were included in these studies. I also looked at the babies dead in the middle
or 0 z-scores.
For doubling the birth weight the range was:
13 1/2 weeks for female infants born at -3 z-scores to slightly over 4-1/2 months for
those born at +3 scores. That is about a 1 1/2 month spread. At a z-score of 0 the
doubling occurred at just under 4 months.
11-1/4 weeks for male infants born at -3 z-scores to almost 4-1/2 months for those born
at +3 scores. That is about a 2 month spread. At a z-score of 0 the doubling occurred at
3-1/3 months.
For tripling the birth weight the range was:
10 1/2 weeks for female infants born at -3 z-scores to 19 months for those born at +3
scores. That is about a 9 month spread. At a z-score of 0 the doubling occured at just
under 15 months.
10 months for male infants born at -3 z-scores to 16-1/2 months for those born at +3
scores. That is 6-1/2 month spread. At a z-score of 0 the doubling occurred at 13
months.
What are the implications of this? The range of multiple of birth weights is too great to
be useful. How can you possibly make a determination when the range in doubling the
birth weight is great than 2 months during the first six month of life? And it is even wider
at up to a 6-1/2 to 9 month spread when you triple the birth weight.
Statistically (regression to the mean - and enormous range in time to doubling and
tripling), biologically (catch up and catch down growth), diagnostically (providing relevant
info about the recent health history), and behaviorally (activity patterns change before
health and growth are compromised) --- use of doubling or tripling the birth weight is of
little value.
Best, Susan Burger
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