A study of fetal growth retardation in teratological tests: Relationship between body weight and ossification of the skeleton in rat fetuses

The normal development of preterm rat fetuses (day 19.0 to day 21.0) was investigated with respect to body weight and ossification of the sacrococcygeal vertebrae, supraoccipital bone, sternebrae, and proximal phalanges in the forepaw. Normal standard growth curves were established for these indexes in rat fetuses. A method was devised for the analysis of the relationship between low body weight and retarded ossification induced by teratogenic agents. In normal fetuses, mean body weight and number of ossified sacrococcygeal vertebrae and the ossification stage of the supraoccipital bone increased approximately linearly with advancing fetal age. The number of ossified sternebrae and proximal phalanges in the forepaw increased curvilinearly with advancing fetal age. By use of the standard curves, fetal growth retardation observed in teratological experiments may be expressed as retardation in relation to the standard in hours. The characteristic pattern of growth retardation induced by an agent may be evaluated by comparison of the degree of retardation expressed in a common “hour” scale among various growth indexes. This method was applied to experimental data of fetal growth retardation induced by maternal fasting. The degree of retardation was found to differ among growth indexes; body weight and ossification of the sacrococcygeal vertebrae were the most severely retarded; ossification of the supraoccipital bone and the sternebrae was moderately retarded; ossification of the proximal phalanges in the forepaw was least retarded.