AbstractOpitz has defined developmental field defects (DFD) as “any dysmorphogenetically reactive unit of the developing organism that leads to final structure.” We have incorporated in our coding system specific codes to identify individual DFDs in each child, irrespective of the cause or type of the MCA pattern (i.e., chromosomal, mendelian, environmental, or unknown). Using this approach, we can analyze the group of defects included in the expression of each DFDas a discrete unit.To confirm our hypothesis that vertebral and rib anomalies constitute a DFD, we have studied all of our cases with segmentation anomalies of the spine and ribs, including hemivertebrae, fused or absent vertebrae, and “crab‐like” thorax, all of which fall into the loosely defined groups of spondylothoracic dysplasia, costovertebral dysplasia, Jarcho‐Levin syndrome, and others. The study was performed using the 18,743 malformed children from the ECEMC data files, 110 of them having vertebral/rib anomalies. © 1994 Wil