Jaundice is caused by an excess of bilirubin. Bilirubin is found in the blood but usually only in very small amounts. In the newborn period, bilirubin can build up faster than it can be removed from the intestinal tract. Jaundice can appear as a yellowing of the skin and eyes. The jaundice usually clears up by two weeks of age and usually is not harmful.

Some breastfed babies develop jaundice when they do not get enough breastmilk, either because of breastfeeding challenges or because the mother's milk hasn't come in. This type of jaundice typically clears up quickly with more frequent breastfeeding or feeding of expressed breastmilk or after the mother's milk comes in.

Your baby's doctor may monitor his or her bilirubin level with blood tests. Some babies with jaundice may need treatment with a special light (called phototherapy). This light helps break down bilirubin into a form that can be removed from the body easily.

Keep in mind that breastfeeding is best for your baby. Even if your baby gets jaundice, this is not something that you caused. Your doctor can help you make sure that your baby is eating well and that the jaundice goes away.

source: http://www.womenshealth.gov