It’s often said that breastfeeding (particularly while lying down at night) will cause tooth decay, just like letting a baby sleep with a bottle of milk can cause “baby bottle mouth.” Essentially, a valid link has not been made between breastfeeding (nighttime or otherwise) and cavities.
Before the use of the baby bottle, dental decay in baby teeth was rare. Two dentists, Dr. Brian Palmer and Dr. Harold Torney, have done extensive research on human skulls (from 500-1000 years ago) in their study of tooth decay in children. Of course these children were breastfed, probably for an extended length of time. Their research has led them to conclude that breastfeeding does not cause tooth decay.
One of the reasons for nighttime bottles causing tooth decay is the pooling of the liquid in baby’s mouth (where the milk/juice bathes baby’s teeth for long periods of time). Breastmilk is not thought to pool in the baby’s mouth in the same way as bottled milk because the milk doesn’t flow unless the baby is actively sucking. Also, milk from the breast enters the baby’s mouth behind the teeth. If the baby is actively sucking then he is also swallowing, so pooling breast milk in the baby’s mouth appears not to be an issue.