Annie of SA Parent Life has a wonderful compendium of information from multiple sources about the benefits of breastfeeding for both mother and child. World Breastfeeding Week runs from August 1 through August 7.
Initiating the oxytocin bond between mother and child, and beginning the process of brain development whereby the baby will come to associate the warmly connected feeling of oxytocin with safe intimacy.
In addition, according to Annie's blog, allowing the baby to drink colostrum, the first milk, provides crucial nutrition and immunity. She also includes a link to video of the "breast crawl," the instinctive movement of a newborn toward the breast.
See also "The Mother/Baby Attachment Gap"
