Excerpt from Natural Baby and Childcare

Not all carriers are created equal. Nowadays, I see more parents "wearing" their baby; however, many are using upright baby carriers, or harness. You may be surprised to learn upright carriers can be harmful to your baby's spine, leading to back problems. Your baby is born with two normal curves: middle back and base. (The adult spine has four normal curves: neck (cervical spine), middle back (thoracic spine), lower back (lumbar spine), and base (sacrum).) In the popular upright carrier, in which your baby is harnessed with her legs dangling from either side, her body weight is being held in her pelvis at the base of the spine.

As with baby walkers and jumpers, the upright infant carrier places your baby in a position where the spines bears all of her weight, a posture your baby is not physically ready to handle. This can influence her back development, and according to chiropractor Rochelle L. Casses, sometimes "cause[s] spondylolisthesis (a painful back condition in which the vertebra of the lower back slips out of place). In addition, it places baby's legs in a bowlegged position, and can hinder circulation." The upright vertical carrier can also lead to a whiplash type of injury in the neck if used before a baby has proper head control. Moreover, the front carriers do not allow for a baby to change position. Given the risk of lower back conditions, I strongly urge you to avoid the upright infant carriers, as well as baby walkers and jumpers that place the weight on the base of the spine.