How to calculate percentile weight loss for infants

Updated November 22, 2016

As any mom can tell you, babies grow quickly. But during the first week or so of life, babies actually lose weight. A healthy formula-fed baby will normally lose about five per cent of his birth weight, while a breastfed baby averages seven per cent in weight loss. (Ref. 1.) A mother needs to keep track of the baby's weight changes during the first few weeks to be sure that the baby is healthy and getting enough to eat.

Make sure that your newborn is weighed carefully and accurately immediately after birth. If you have any doubts about the measurements your doctor gives you, ask to have the baby weighed again. Knowing the baby's birth weight is essential for calculating weight loss and the amount of time it takes your baby to regain its birth weight.

Ask to have your baby weighed when you are discharged from the hospital or birth centre. This weight will be lower than the weight the baby had at birth. Doctors and scientists are not quite sure why babies lose weight during the first few days after birth, but almost all babies do.

Calculate the per cent of weight loss. Divide the lower weight by the baby's birth weight. This will give you a decimal amount.

Monitor your baby's weight---either at home or by taking baby into the paediatrician's office for weight checks. Within a week, the baby's weight should begin to increase. In most cases, babies regain their birth weights within two weeks, though some take as long as three weeks. If your baby loses more than 10 per cent of his birth weight or continues to lose weight after a week, you need to talk to your paediatrician. If you are breastfeeding your baby, you will need to meet with a lactation consultant to make sure that your baby is nursing properly and that you have an adequate milk supply.


Speak with a lactation consultant before you quit breastfeeding altogether. Some babies need a little encouragement to get them eating properly. If you need to supplement with formula, the consultant will teach you ways to offer formula without giving the baby a bottle.

Things You'll Need

  • Scale
  • Calculator
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About the Author

R.J. Bowman has a Bachelor's degree in accounting with a minor in English from Pensacola Christian College. After college, she taught English to seventh graders until becoming a mom. At that time, she found freelance writing to be a great way to keep her writing skills sharp.