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The human skull is normally composed of multiple bones, and in infants, these bones are not yet fully fused. This results in gaps known as fontanelles or soft spots. Imagine them as puzzle pieces that have not been completely joined. This structure allows the infant's skull a remarkable degree of flexibility which is essential for various reasons.
The flexibility provided by fontanelles enables the skull to change shape slightly as the baby moves through the birth canal. Without this flexibility, the rigid skull would make childbirth significantly more challenging, posing risks both to the mother and the baby.
Furthermore, this flexible arrangement is not just practical for a smooth delivery but also primes the infant for their rapid growth and development phase after birth.

As labor progresses, the baby’s head must navigate through the constricted passage of the birth canal. Normally, this passage is too narrow for the rigid skull of a newborn.
This is where the existence of fontanelles becomes crucial, as they allow the skull bones to overlap slightly. A process known as "molding" takes place, where the flexibility of the skull helps in reshaping it to fit through the birth channel.
Such adaptability ensures a safer passage through the birth canal, reducing trauma risks. Success in this process largely depends on how well the bones can shift and compress without causing harm, and fontanelles are key players in this intricate balancing act.

Upon birth and during the early months of life, a newborn’s brain undergoes rapid growth and development. The presence of fontanelles plays a vital role during this period.
They provide enough space for the brain to expand comfortably. While the brain is growing explosively, these soft areas act as a safety net, allowing expansion without immediate restrictions of a tightly fused skull.
This isn't just about growth; it's about protecting the brain as it develops its many complex functions. Over time, as the brain approaches its growth milestones, the fontanelles gradually harden and fuse into solid bone, marking the end of a critical phase of brain development.