Until now, weight loss during the first 3 to 4 days after birth has been considered one indicator of how early breastfeeding is going.  If on Day 4 a newborn’s weight loss is in the average range of 5% to 7%, this usually means breastfeeding is going well.   Nearly all babies lose some weight after birth, because after floating in amniotic fluid for 9 months, they are born waterlogged.  Normal weight loss comes from the shedding of this excess fluid as they adjust to life on the drier outside.

But when babies lose more than 7% of birth weight during these early days, does this automatically mean they are not getting enough milk?  No, according to a recent study.

A greater weight loss may be completely unrelated to breastfeeding and due instead to excess IV fluids mothers receive within the final 2 hours before delivery.  According to this study, these excess IV fluids inflate babies’ birth weight in utero and act as a diuretic after birth.  Babies whose mothers received more IV fluids before birth urinated more during their first 24 hours and as a result lost more weight.  Number of wet diapers during the first 24 hours predicted infant weight loss.  This was true whether the babies were born vaginally or by c-section.  Another study published earlier this year had similar findings.

This weight loss has nothing whatsoever to do with breastfeeding and milk intake.  In fact, the authors suggest that if clinicians want to use weight loss as a gauge of milk intake, they calculate baby’s weight loss not from birth weight, but from their weight at 24 hours.  According to their findings, this could neutralize the effect of the mother’s IV fluids on newborn weight loss.

This is one more reason weight loss alone should not be used to determine when newborns need formula supplements.   The Academy of Breastfeeding Medicine put this well in one of its protocols: “Weight loss in the range of 8-10% may be within normal limits….If all else is going well and the physical exam is normal, it is an indication for careful assessment and possible breastfeeding assistance.”

http://www.nancymohrbacher.com/articles/2011/10/31/newborn-weight-loss-and-iv-fluids-in-labor.html