A baby is born with approximately 270 to 305 bones, commonly cited as around 300 bones at birth. This number is higher than the 206 bones found in adults because many of a baby's bones are initially separate and made partly of soft cartilage, which allows flexibility for growth and movement during birth and early development

. As the baby grows, many of these bones gradually fuse together through a process called ossification, where cartilage is replaced by bone and smaller bones join to form larger ones. This fusion reduces the total number of bones to the typical adult count of 206 by around the age of 20

. In summary:

• Number of bones in a newborn baby: about 270 to 305 (commonly ~300)
• Number of bones in an adult: 206
• Reason for difference: fusion of bones and ossification of cartilage during growth