Obstetrical implications of homeotic transformation of vertebrae and assimilation of coccyx with sacrum in humans

Assimilation of coccygeal vertebrae with the sacrum can be an obstetrical impediment in humans. This study evaluates homeotic transformation of the thoracic-lumbar (T-L) and lumbar-sacral (L-S) boundaries and position of the diaphragmatic vertebra (DV) as proximate determinants of this assimilation. Sample is 311 females and 791 males, ages 20-49 years, who died in the United States from the 19th to 21st centuries. Results show 33% of females and 42% of males have fusion of coccygeal vertebrae with the sacrum. In both sexes, cranial transformation of the T-L and L-S boundaries is associated with assimilation of coccygeal vertebrae with the sacrum, whereas caudal transformation of these boundaries is associated with inhibition of this assimilation. Males, but not females, also show assimilation of coccygeal vertebrae with the sacrum with cranial transformation of DV, and inhibition of this assimilation with caudal transformation of DV. Females have higher prevalence of caudal transformation of the T-L and L-S boundaries than males (15% and 7%, respectively), and males have higher prevalence of cranial transformation of these boundaries than females (22% and 11%, respectively). Assimilation of coccygeal vertebrae with the sacrum contracts posterior space of the pelvic outlet. Dual contraction of posterior and anterior spaces of the outlet can be obstetrically perilous. High prevalence of assimilation of coccygeal vertebrae with the sacrum contributes to selection for sexual dimorphism of the pubic arch and angulation of the sacrum, with females larger than males.