Jaundice is a little tough, because, like many eloquent posters have noted, it is very common, and is often harmless. I am a pediatrican, soon to be neonatologist, so I treat this all the time. The jaundice that occurs due to blood group incompatibilities is just as a PP said, mom gives baby antibodies that break down baby's blood cells, and jaundice occurs. This type usually happens sooner (like in the first 24-28hrs) than the physiologic kind (3-4 days). I agree that in general, a jaundiced 4 or 5 day old who is feeding well, peeing and pooping several times a day, is in no danger. However, a baby jaundiced in the first day or two, or who is jaundiced to the legs, or who is really sleepy, not feeding, peeing, pooping, etc, should probably be checked. The danger is rare, but its irreversible brain damage from bilirubin staining the brain tissue. Light therapy, and (rarely needed), exchange blood transfusions, can prevent that. However, there is no need to give formula during phototherapy, breast milk works just great, anything to get those little bowels working. At my hospital, we encourage nursing, or, if the baby is too sleepy or can't latch, we will offer tube feeds of mother's milk, if the parents prefer. Or gentle use of IVs, in the case of a truly dehydrated baby. A baby who has been jaundiced for more than a week or two, even if healthy, should also have some blood tests, because some liver diseases (biliary atresia, etc), can present this way, and early treatment is better. One of the best things these days are transcutaneous bilimeters, which can check the bili using light, without a heel poke. Hopefully, these will become the standard, they are pretty accurate. Hope not to offend with my post, just one poor doc's experience.