The development of baby’s spine to the typical S-shape usually takes a bit more than one year and can be divided in four stages.
When a baby is born the spine is in total kyphosis which means it’s rounded in the shape of a C. None of the muscles which help straighten the spine are strong enough yet.
The first part of the spine to be straightened is the uppermost part, called cervical region. When a baby learns to hold up his head, usually around 3-4 months, the curve (kyphosis) of this part slowly gets straighter and finally slightly curved the other way (cervical lordosis).
When a baby learns to sit up, usually around 9 months, the muscles in the middle of the spine (thoracic region) get stronger and help supporting the baby’s back. The spine in this part stays rounded (thoracic kyphosis).
Finally the lower back (lumbar area) gets straightened and curved the other way (lumbar lordosis). This stage is finished when a child has learned to walk, usually around 12-18 months. The typical S-shape has been reached, as the child’s muscles are strong enough to hold up the whole back.
What does that mean for carrying your baby?
The younger a baby is the more important it is to support the spine on it’s whole length because there are not enough muscles to support the back and also the intervertebral discs cannot act as shock absorbers yet. The discs look like jellified doughnuts and when the spine is at rest in an optimal position they puff out. They will increase in thickness until the age of two when they make up one quarter to one third of the length of the spine and are a major determinant of your adult height.
A good sling therefore supports the baby’s back evenly like a firm bandage so his back cannot slump in it, allowing the intervertabral discs to puff out, and increase in thickness. The slings material needs to be soft enough and not too stiff to achieve this effect, and the sling needs to be adjustable to offer support as needed.
Good support of a rounded back is also important for older babies when they fall asleep because then their muscles relax and the sling needs to compensate this.
So in short; your child could be taller if they are carried, then if they aren’t.