Maternal toxicity--a possible factor in fetal malformations in mice.

The assumption that major fetal malformations are indicative of a chemical's teratogenic potential was not supported by a literature review of teratology studies conducted in mice. In these studies, dose levels of test agents that manifested maternal toxicity as suggested by reduction in dam's body weight, clinical signs of toxicity, or deaths, also invariably caused reduction in fetal body weight, increased resorptions, and rarely fetal deaths. In several such studies, conducted with maternotoxic doses of structurally unrelated test agents, a consistent pattern of fetal defects was discovered. These defects included exencephaly, open eyes, hemivertebrae, fused arches or centra of lumbar or thoracic vertebrae, fused, missing or supernumerary ribs, and fused or scrambled sternebrae. These defects were absent at drug dosages that were distinctly nontoxic for the mother. In a few studies conducted at two or more maternally toxic doses, the degree and severity of maternal toxicity showed a positive correlation with the incidence and severity of above fetal defects. It is hypothesized that maternal toxicity, on its own, may have an etiologic role in these fetal defects.