The ossification of the costal element of the seventh cervical vertebra with particular reference to cervical ribs.

During 16 years of study at Northwick Park Hospital 715 fetuses were radiographed to determine the incidence of skeletal malformations. A technique using low kV and industrial film provided higher resolution radiographs than would have been possible in live births because of dose considerations. Gestational age calculated from LMP and ultrasound findings ranged from less than 18 weeks to 42 weeks. A separate costal element of C7, the possible precursor of a cervical rib, could be identified in 63% of fetuses. Males outnumbered females 403:311 but there was no intersex difference in the presence of such rudimentary cervical ribs with 63 and 64% respectively in each group. Of the 452 cases with separate costal elements, 73% were bilateral. Of the unilateral group 60% occurred on the right and 40% on the left. There was no significant difference in the incidence between the gestational age groups from 20 weeks onwards, confirming that the disappearance of the separate element occurs predominantly after birth, 394 cases (87%) of fetuses had an otherwise normal skeleton.