Models to predict normal fetal growth have been of limited accuracy. Abnormal growth is even more problematic. We have developed a mathematical modeling system based on observed body and brain weights to study fetal growth patterns in midgestational trisomy 18 fetuses. Third-degree polynomial-based analysis using observed fetal body and brain weights to generate predicted weights for various organ systems allows for the comparative study of growth patterns at various fetal weights and gestational ages. Our data suggest that what has been previously called symmetrical intrauterine growth retardation associated with aneuploid fetuses is really an asymmetrical pattern that is dynamic in nature and may change through the course of pregnancy.