Babies are typically born without fully developed kneecaps (patellae). Instead of hard bony kneecaps, they have a structure made of cartilage that gradually ossifies and hardens into bone between about 2 to 6 years of age. This cartilage allows for easier passage through the birth canal and provides flexibility as the baby grows and learns to move.

Additionally, some babies can be born with congenital limb defects where parts of limbs may be missing or underdeveloped, but this is not typical for all babies and is considered a birth defect.

In rare and severe birth defects such as anencephaly, babies are born without parts of the brain and skull, but these are medical conditions rather than normal developmental features.

In summary, the normal body part that babies are born without as bone, but instead have as cartilage, is the kneecap (patella).