Here is something I wrote up for our nurses regarding the math and assumptions around pre and post feed weights.  Maybe it will help this dad?

"Pre and Post Feed weights can be a useful tool when done properly but there are so many variables and assumptions that go into the calculation from weight to liquid intake that without great care they are useless at best and harmful at worst.  
First the math:  The conversion we are making is g to mL(cc)  
		This conversion is made using density, which has units of g/mL
		To convert we take the gram weight from the scale and divide by g/mL
Water has a density of 1.00g/mL so we can make a direct conversion, the gram weight cancels the g density and we just get mL.

The assumption we make with pre/post feed weights is that human milk has the same density as water, which it certainly does not.  I can't recall the exact density but let's just say that it is 1.016g/mL.  Now if we get a pre/post gain of 50g we must divide by 1.016 and get an intake of 49.21 mL, not a huge difference in this case but what if the weight gain was only 10 g?  These calculations can be done for pound to fluid ounces but it is much easier and more accurate in metric.

Now the uncontrollable variables:
	Between the time we do the pre-weight and the post-feed weight the baby may poo, pee and spit up, these are all visible weight losses.  Also there will be "insensible" water loss through perspiration and respiration.  If not done with these things in mind a pre/post weight loss is not uncommon.
	To control for these variables the weights should be done with the baby lightly clothed (to allow close contact with mom) and wrapped in a receiving blanket (to try to capture the majority of the insensible water loss and spit up).  

Generally we'll want a naked weight on any baby that we are following closely enough to want a pre and post feed weight so that should come first so that the result can help to inform the rest of the visit.  The baby should then be clothed as above, the scale set to grams and the pre-feed weight taken and entered.  Post-feed weight is then taken using the "reweigh" button.  The BabyWeigh scale will then calculate the difference in grams, this can be directly converted to mL recognizing the density assumption noted above."

Janie Akerlund RN IBCLC
Nurse-Midwifery Birth Center
Eugene, Oregon


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