There are four stages to your baby's spine development; this blog is about the first stage; the cervical spine.
At birth,your baby's spine is shaped like the letter C. This is the primary curve. As your baby moves about, his spine develops secondary curves. Secondary curves are necessary so that our spines can balance/distribute the weight of our body. An adult spine has two secondary curves that give it an S-like shape.
As your baby begins to lift his head, the muscles of the neck become stronger. The strengthening of the muscles against gravity are what help to change the shape of the cervical spine (the first seven vertebrae). Cervical spinal development is complete when your baby can rest on his forearms and hold his head up without assistance as seen is the diagram.
But before completion of this stage of spinal development, it is of particular importance to ensure that you support your baby's head. And when using a carrier, your baby is in a position that is safe and supportive. Although some suggest using a carrier in the cradle position (the manner in which to breastfeed a baby), it is not the safest way to carry your child. The cradle position could restrict your baby's airflow and the position does not support hip development.
To carry your baby in the most comfortable and safest way, wear your baby facing you, vertically, with his head resting on your heart and with his chin off of his chest. By helping him to keep his chin off of his chest, it is a sure way to know his airway is open.
Vertical positioning not only offers your baby a safer option but it requires your baby to balance his head against gravity which strengthens the neck muscles required for sitting. Vertical positioning also helps with the stimulating the vestibular system, the system responsible for balance, proprioception, coordination, muscle tone and more.