Hello. Welcome to Healthcare Magic. First, congratulations on the new addition to your family. Although birth trauma is occasionally the cause of unusual postures in newborn infants, these issues are often a sign of a developing neurological deficit that had its beginnings before birth. For example, neck hyperextension, especially when it is associated with shoulder shrugging, is sometimes an early sign of cerebral palsy. On the other hand, neck hyperextension may just be a manifestation of weak neck muscles, and this condition often corrects over time. Your pediatrician is the first person to see about this problem. If it s warranted, he/she can refer you to a pediatric neurologist. I hope things go well!