If your baby is given bottles too early, before he learns to breastfeed well, he may be at risk for nipple confusion which may in turn put your milk supply at risk for never becoming fully established and result in your baby's frustration at and perhaps rejection of the breast.
During breastfeeding a baby's tongue, jaw, and mouth work together in a coordinated rhythm. This unique sucking action is one reason why breastfed babies overall have better oral development than bottlefed babies. Once the baby latches on, the tongue comes down and out as it cups the breast. The lips must be flanged out resembling a rose petal or a fish's lips. Letdown can sometimes takes several seconds to perhaps more than a minute to occur. The baby learns that he does not get an instant reward; he must "work" for mother's milk.
With bottlefeeding, the baby is instantly met with a flood of milk as a bottle will allow milk flow without active sucking. This sudden gush forces the baby to flip his tongue upward to help regulate the flow and prevent him from choking. His lips are pursed tightly around the firmer artificial nipple and no work is required of his jaws.
One study found that 95% of babies will become confused if given a bottle during the first 3-4 weeks of life.
For some babies it may take many bottles before they show any nipple confusion; for others it can take only one or two. For this reason it is best to avoid offering your baby a bottle before he is 4 weeks of age.