A study of fetal growth retardation in teratological tests: an examination of the relationship between body weight and ossification of coccygeal vertebrae in mouse and rat fetuses.

We examined the relationship between body weight and ossification of coccygeal vertebrae in normal full-term fetuses of mice and rats to reliably evaluate fetal growth retardation in teratological tests. Correlation coefficients between body weight and number of ossified coccygeal vertebrae were positive in mice and rats. However, the coefficients were variable between groups and were not statistically significant in some groups. Averages of body weight and ossified coccygeal vertebrae were variable in the normal fetuses. However, the coefficient of variation in each group was nearly constant, and the component of variance between groups (sigma 1(2)) was small as compared with that within groups (sigma 0(2)). Therefore, the relationship between low body weight and retardation of ossification observed in teratological experiments may be evaluated by measuring relative differences of body weight and ossification between the treated group and the control group with the variance within groups (sigma 0) as a scale.