Any time you give your child cow's milk, that's a replacement for your milk. Your milk is much better for your baby. (unless, of course, your baby is a cow!) Introducing cow's milk is a method of weaning, which is okay if that's what you're wanting to do. My daughter is 18 months and has never had any cow's milk, although she does eat some cow's milk products (yogurt, cheese). There is nothing magic about cow's milk for humans--there are many other ways to get calcium. Sounds like your doctor needs some up to date information about breastfeeding!