The human body undergoes a continuous developmental process that does not pause for birth. While a newborn appears fully formed, many internal structures are in a state of functional or anatomical immaturity. These “missing” parts are often present in a different, more flexible form, such as soft tissue instead of rigid bone. Other structures exist as temporary fetal components that regress once their purpose is fulfilled. This design facilitates the birthing process and accommodates the rapid growth that characterizes infancy.
Skeletal Structures That Are Not Yet Bone
The skeletal system of a newborn differs significantly from an adult’s, containing about 300 bones compared to the adult’s 206. This is because many future bones exist as cartilage or have not yet fused. This flexible skeleton is necessary for navigating the narrow birth canal and accommodating the dramatic growth of the brain. The process of turning cartilage into hardened bone is called ossification, which is actively underway at the time of birth.
The kneecap, or patella, is a prime example of a structure that is entirely soft cartilage in a newborn. This bone does not typically begin to calcify and form bone until a child is between two and six years old. Until then, the soft patella provides flexibility and protection to the knee joint, which is advantageous for infants who are not yet walking.
The top of the skull also features large gaps, known as fontanelles, or “soft spots,” where multiple skull plates have not yet fused. These fibrous membranes allow the skull bones to overlap slightly during birth, preventing head injury. The fontanelles also permit the brain to undergo massive expansion during the first year of life. The posterior spot closes within a few months, while the larger anterior one remains open for 18 months or longer.
Facial and Oral Structures That Are Undeveloped
The newborn face and oral cavity show signs of immaturity, particularly in structures related to future functions like chewing and respiration. Although babies are born without visible teeth, the foundation for their primary teeth is already present deep within the gums as developing tooth buds. Eruption, where the teeth push through the gums, typically begins around six months of age. The full set of primary teeth is usually not completed until around two to three years old.
The paranasal sinuses are largely undeveloped at birth, existing only as tiny cavities. While the ethmoidal and maxillary sinuses are present in rudimentary form, the frontal and sphenoidal sinuses are often only just beginning to form or are completely absent. These air-filled cavities begin their major expansion after birth, gradually growing to their adult size as the facial cranium develops. This process continues into adolescence.
Newborns frequently appear to cry without producing actual tears immediately after birth. This occurs because the lacrimal ducts, which drain tears into the nasal cavity, are often not fully open. While the tear-producing glands are functional, a small membrane at the end of the duct may temporarily block the drainage path. This obstruction usually resolves spontaneously within the first few weeks or months of life, allowing for the full flow of visible tears.
Structures That Close or Disappear After Birth
In contrast to immature structures, the circulatory system contains specialized features necessary for fetal life that become obsolete immediately upon taking the first breath. These structures are designed to functionally “disappear” as the body switches from relying on the placenta for oxygen to using the lungs. Two major fetal circulatory shunts are responsible for rerouting blood away from the non-functional fetal lungs.
The Foramen Ovale is a small, flap-like opening between the right and left atria of the heart. It allows oxygenated blood from the placenta to bypass the lungs and move directly into the systemic circulation. When the newborn takes their first breath, the pressure in the left side of the heart increases, causing this flap to press closed. This functional closure is typically immediate, though the anatomical sealing takes longer.
Another bypass is the Ductus Arteriosus, a blood vessel connecting the pulmonary artery directly to the aorta. This vessel also shunts blood away from the fetal lungs. After birth, a rise in oxygen levels causes this muscular vessel to constrict and close, typically within the first few days of life. Once these shunts close, the entire output of the right ventricle is directed to the expanded lungs for oxygenation, completing the transition to the adult circulatory system.
The umbilical cord itself contains structures that regress and become internal ligaments once the cord is clamped and cut. The umbilical arteries and the umbilical vein carry blood to and from the placenta during gestation. These vessels are no longer needed for circulation and atrophy, transforming into fibrous cords.