A radiographic study of the human fetal spine. 1. The development of the secondary cervical curvature.

The present study has shown that the secondary curvature in the cervical spine of the human fetus develops at a much earlier age than has been thought. It may, in fact, develop soon after the embryo first acquires a neck and begins to uncurl. The early appearance of this curvature may be related to the early development of function in the muscles responsible for head extension, this movement being a basic component of the primitive "gasp" reflex. It is further suggested that, although the embryo develops in flexion, the traditional assumption that such flexion is a consequence of fetal musculoskeletal development must be questioned, and the expression "normal fetal flexed position" is probably misleading, since it does not take into account the wide range of movements of which the fetus is capable.