See http://www.boba.com/our-position-on-position and http://welladjustedbabies.com/are-baby-carriers-safe/
Being in a supported upright position, as in babywearing with the baby facing the parent, is a GOOD idea for many, many reasons.
- The vestibular system gets developed by the baby responding to the parent's movements and turns;
- Hips are kept within their sockets;
- Good for temperature regulation;
- Spine is not pressed flat as it would be while lying down;
- Head is not flattened against any surface;
- Several others.
If the baby faces forward, the spine may be forced into a convex position.
Lying down flat may force the head and spine into unnatural positions.
Immobilizing the legs, especially in a straightened position, negatively affects development of the hip joint and muscles, thus fostering bad posture. In particular, papoose-style carriers force the knees to straighten, which is an unnatural position for the baby. The hip joint is best developed if the knees are allowed to bend toward the body and the legs are not immobilized.