Newborn jaundice is normal in most cases, appearing within 2-3 days post-birth, and affecting up to 60% of full-term babies.
Physiologic jaundice is caused by a buildup of bilirubin, which is produced when red blood cells are broken down. The liver is responsible for eliminating the bilirubin, but a newborn’s liver is often too immature to efficiently handle this process yet. This causes a yellow cast on the skin (which can be trickier to detect in dark-skinned babies) and the eyeballs. This resolves itself in a week or two as the baby matures further and red blood cell levels have lowered.
In a breastfed baby, jaundice is more common and tends to persist longer than a formula-fed baby (as breastfed babies are the standard, this means it’s the norm). True breastmilk jaundice, which only affects 0.5% to 2.4% of newborns, sticks around longer than one or two weeks, sometimes up to twelve (now, this shouldn’t be confused with breastfeeding jaundice, which is caused by starvation/lack of proper milk intake). Bilirubin levels might even increase at the two-week mark. None of this is a cause for concern in an otherwise healthy baby.
On how breastmilk and formula compare in causality of newborn jaundice, Dr. Sears says:
“The difference is thought to be due to an as-yet unidentified factor in breastmilk that promotes increased intestinal absorption of bilirubin, so that it goes back into the bloodstream rather than moving on to the liver. Higher rates of jaundice in breastfed infants may also be related to lower milk intakes in the first days after birth, because of infrequent or inefficient feeding.”
Hence, why medical treatments should be avoided unless truly necessary because they threaten to interrupt breastfeeding further. As breastmilk helps move the baby’s bowels to remove excess bilirubin, frequent feedings will hasten the normal bodily process.
If bilirubin levels have reached more than 20 milligrams, a health provider might recommend treatment with phototherapy (ask about fiber optic blankets, an especially good option for nursing moms).
What not to do: do not supplement with sugar water, and do not restrict the baby from breastfeeding.
Links:
Mama's Milk, No Chaser 