"Why do they say not to give a newborn baby a bottle if you are breastfeeding?"

Giving a healthy baby a bottle before 4 weeks old can interfere with successful breastfeeding. Formula or glucose water may fill the baby and make him/her lose interest in breastfeeding. If plain water is given, the new baby is more likely to develop jaundice. Babies who are prone to allergies are more likely to develop a cow's milk allergy if they receive formula as a newborn. Breastfeeding requires a different type of suck than bottle feeding and there is no such thing as an artificial nipple that is just like breastfeeding. Babies can get confused if they get a bottle while learning how to breastfeed and may refuse the breast. Supplemental bottles may lead to engorgement (painfully full breasts) because the baby won't be feeding as much. Less time at the breast also interferes with developing a good milk supply. Source: The Breastfeeding Answer Book, LaLeche League International, 1997