Calculating Percent Weight Loss
Knowing your newborn’s percent weight loss is critical to ensuring adequate feeding. Excessive weight loss, especially greater than 7 percent of the birth weight has been associated with higher rates of excessive jaundice (or hyperbilirubinemia) and high sodium dehydration (or hypernatremic dehydration), both of which can impair brain development.[1,2]
You should be able to ask your health provider what your baby’s percent weight loss is; but if you are home weighing your own baby you can calculate your baby’s percent weight loss yourself.
The following is a step-by-step guide to weighing your baby and calculating their percent weight loss.
How to Calculate Your Baby's Percent Weight Loss
Babies should be weighed naked. The first step is to know your baby’s birth weight in grams (g), not pounds and ounces. With each subsequent weighing after birth, percentage of weight loss can be calculated in the following manner:
[Birth weight – current weight] divided by birth weight x 100% = percent weight loss
Step 1: Birth weight (in g) minus current weight (in g) = difference in weight
Step 2: Divide the difference in weight in Step 1 by the birth weight (in g)
Step 3: Multiply the value in Step 2 by 100%, which will give you percent weight loss
Step 4: Note how many hours have elapsed since birth and plot the percent weight loss on the appropriate Newborn Weight Loss Tool graph shown in the Appendix.
(1 ounce = 28 grams and 1 pound = 16 ounces = 454 grams)
See the example below:
If the birth weight is 3340 g and weight at 24 hours is 3127 g,
Step 1: 3340 g – 3127 g = 213 g
Step 2: 213 g ÷ 3340 g (birth weight) = 0.0638
Step 3: 0.0638 x 100% = 6.38 % weight loss at 24 hours (which is between the 75th and and 90th percentile on the NEWT. Any result that is 75th percentile or above is concerning and requires full evaluation and most likely supplementation.
Alternatively . . .
You can go to the following online calculator and enter your baby’s data:
For more accurate information:
Go to the Newborn Weight Loss Tool and enter your baby’s birth weight and any subsequent weights along with the date and times they were measured to assess your baby’s nutrition. Greater than 75th percentile percent weight loss (below the yellow line) is considered excessive weight loss and is an indication for comprehensive evaluation of your baby’s hydration and nutritional status by your baby’s doctor, as well as your breast milk supply and how well your baby transfers milk (through a weighted feed) by an IBCLC.
If the evaluation indicates underfeeding or you are unable to get immediate evaluation (within minutes), supplementation with expressed breast milk, donor milk, or formula until your baby is satisfied can prevent medical complications.
References
- Wen-Chieh Yang et al., “Bodyweight Loss in Predicting Neonatal Hyperbilirubinemia 72 Hours after Birth in Term Newborn Infants,” BMC Pediatrics 13 (2013): 145, https://doi.org/10.1186/1471-2431-13-145.
- Mónica Ferrández-González et al., “Weight Loss Thresholds to Detect Early Hypernatremia in Newborns,” Jornal De Pediatria, July 18, 2018, https://doi.org/10.1016/j.jped.2018.06.005.