Weight loss patterns vary widely, due to the many circumstances surrounding birth—length of labor, type of birth, administration of IV fluids, use of pain medication, maternity care practices, and more. One systematic review of 11 studies found that newborn weight loss ranged from 3.7 percent to 8.6 percent. Most babies started gaining weight by day 4 and were back to their birth weight by day 9.
Unfortunately, weight loss studies, including those in this systematic review, are rife with limitations:
- Day of birth may be counted as either day 0 or day 1.
- Weight measurements on days 1–14 are often incomplete.
- There is no clear definition of “exclusive breastfeeding.”
- There is no clear indication of when slow gaining infants were supplemented with formula.
- Convenience samples are used rather than random samples.
- Only one study followed the infants for a full 14 days.
What remains unclear is what amount of weight loss is "normal" in exclusively breastfed babies, and what is an unintended outcome—the consequence of a myriad of policies, procedures, and practices, which have been shown to be unsupportive of breastfeeding.
For example, a 2011 study indicated that administration of IV fluids during labor and childbirth can artificially inflate the birth weight. Since this is typically used as the baseline figure when calculating weight change, this might lead parents and health care providers to conclude their babies’ weight loss is more worrisome than it truly is. For this reason, those researchers suggested using the baby’s weight at 24 hours—after the baby’s body has had time to excrete the excess fluids—as the basis for weight change.
A generally accepted rule of thumb is that a baby can be expected to lose about 3–7 percent of their birth weight during the first 5 days after birth. They should expect their baby to return to her birth weight by about day 10.
However, a 2016 study of weight change in nearly 150,000 healthy babies found that it is “not uncommon” for babies to be below their birth weight at 10 to 14 days after delivery. About half of newborns were at or above their birth weight at 9 days (vaginal delivery) or 10 days (cesarean section) of age, but about 14 percent of babies born vaginally and 24 percent of those born by cesarean had not regained their birth weight by day 14. For some babies, slower return to birth weight does not indicate a health problem.
Many babies will gain ½–1 ounce (15–30 grams) each day until about 3 months of age. Then, babies typically gain weight more slowly during the remaining months of their first year.
If you are concerned about your baby’s growth or feeding, contact her pediatrician.