The object of this study was to establish new normative data for the fetal rib cage perimeter to thoracic circumference ratio, and to evaluate the potential application of this measurement in the diagnosis of short rib and other skeletal dysplasias. Rib cage perimeter (RCP) and thoracic circumference (TC) were measured in a prospective, cross-sectional sample of 88 patients with normal pregnancies between 14 and 39 weeks of gestation. The RCP and TC were both measured in cross-section, at a level of the four-chamber view of the fetal heart. RCP and TC measurements were also obtained from eight cases known to have skeletal dysplasias. The mean (+/- SE) RCP:TC ratio in normal pregnancies, regardless of gestational age, was 0.670 +/- 0.004. In five of eight cases with skeletal dysplasias the RCP:TC ratio was significantly decreased, and in one case it was increased. The RCP:TC ratio in normal fetuses is independent of gestational age. In this small series the ratio was abnormal in fetuses with certain skeletal dysplasias, and particularly decreased in the fetus with a short rib-polydactyly syndrome. After further evaluation, this technique may prove to be of clinical significance in helping to diagnose skeletal dysplasias, particularly in cases where the gestational age is uncertain.