Babies are born with about 270 bones, but some bones fuse, becoming 206 to 213 as adults. A baby's bones are mostly flexible cartilage, which will harden and become bone over time. You can help your kids develop healthy bones by making sure that they are well-nourished and stay active. Newborns have roughly 270 bones due to a greater amount of cartilage, leading to fewer bones in adulthood as they fuse over time.