At birth, a baby’s spine and hips are still immature. The spine is naturally rounded, in a c-shape, and not designed to be straight. The spine and the hip joints are still soft and flexible.
To support and protect a baby’s developing spine and hips, it is important that the carrier allows the baby to assume the “frog leg” or M position, with the knees higher than the bottom, as shown on the picture below .
This allows for the baby’s back to assume the correct rounded position and for the balls of the hips to sit properly in their sockets. It is also important for the carrier fabric to support the baby’s back. It is not recommended to carry your baby facing out in a sling, as it does not allow the baby to assume the frog leg position and forces the baby's back to be straight.
Sadly, most high street "front pack" type carriers do not provide a good posture for the baby.
In this type of carrier, baby is seated with his legs dangling, with all its weight on his crotch. His back is forced into a straight position. His body is both extended and compressed.
This type of sling also places the baby too low, with the head at mid chest level, which is not a normal carrying position. Finally the design of this type of carrier and the low position of the baby are also not ergonomical for the carrying adult and carrying a heavy baby can quickly become uncomfortable.
For more info, read some articles here and here
