Basics of Baby's Anatomy
Development of the Spine
The development of baby's spine to the typical S-shape usually takes a bit more than one year and can be divided in three 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 back1,2.
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. A good sling therefore supports the baby's back evenly like a firm bandage so his back cannot slump in it. 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.
Development of Baby's Hips
In order to find the ideal way to carry a baby we have to take a look at baby's anatomy. When a baby is born, some of his skeleton is still cartilage and not yet proper bone. Ossification (= cartilage becoming bone) is a process which takes years and is finished when a human being is fully grown up3,4.
One part of baby's body which contains much cartilage is the pelvis. It initially consists of several bones which are held together by cartilage5. In a newborn, where the femoral head (top of the upper leg bone) meets the hip socket (acetabulum), three bony parts of the pelvis are joined together by cartilage and everything is still a bit soft. Also the femoral head is still formed by cartilage. Ossification of the pelvis and the femoral head happen gradually and are usually finished within the first 9 months of life. Especially in this time it's important to aid the development of the hip by supporting correct positioning of the femoral head because wrong positioning, such as tight swaddling, may lead to hip dysplasia6,7.
There is an ideal leg position which centers the femur head right in the middle of the hip socket and thus aids the development of baby's hips. It's described as M-position, frog-position or spread-squat-position and means that baby's knees are higher up than his bottom and that his legs are spread apart ca. 90° (for those with medical background: legs should be flexed min. 100°, abduction should be 30-45°). Babies automatically assume this position when lifted up, because it's also ideal for being carried on the mothers hip. Also babies treated for hip dysplasia using a Pavlik harness or Frejka pillow have their legs in this position6.
A baby carried in the M-position will have a rounded back because the tilt in the pelvis causes the spine to become rounded and viceversa. You can see this in the left-hand photo showing a newborn in a red wrap.
In a cradle carry (baby is lying on her back in a hammock-style sling) a correct M-position cannot be achieved because her legs cannot be spread. So this position is not recommended at all by hip dysplasia specialists8. Instead upright positions, especially on the mothers hip are better for the hip, if a carrier supports correct positioning and a round back.
Here are some pics showing the M-position in a newborn, a 4-month old and a toddler:
A good carrier allows the baby to be carried in the M-position. This is not possible if the fabric in baby's crotch is too narrow (it should support the whole upper leg and reach from knee to knee) or if baby's legs are spread too far apart. So before buying a baby carrier check how a baby sits in there, either by trying it out with your baby or by looking at photos.
Imagine what it would feel like for you to sit in a baby carrier: Wouldn't you prefer to have your whole upper legs supported instead of dangling from your crotch?
8 Dr. Ewald Fettweis: Hüftdysplasie. Sinnvolle Hilfen für Babyhüften; Trias Verlag 2004
© Mirjam Brockmann 2009