Ossification sequence of occipital bone and vertebrae in human fetuses.

The aim of this radiographic study of human fetuses was to examine the pattern and sequence of ossification in the occipital bone and the spinal vertebrae. Together with previous studies of ossification of the human fetal basal cranium, this study can serve as a reference for normalcy in future studies of fetuses with neural tube defects and associated pathological development of the axial skeleton. Thirty-nine normal fetuses aborted between 9 and 14 weeks of gestation were examined. Based on the appearance of ossification centers in the bones under study, the fetuses could be grouped in four well-defined developmental stages, which were named occipito-spinal stages I-IV (OS I-IV). The OS stage was closely related to gestational age, crown-rump length, foot length, and degree of ossification in the hands and feet.