Linear growth in fetuses with trisomy 18 has not been systematically described. We studied the relationship between long-bone, crown-rump, and foot length and gestational age in 17 postmortem fetal specimens with this syndrome. Long-bone and crown-rump lengths were compared with normal regression lines and foot length was compared with gestational age determined on the basis of menstrual dates. Correlation between foot length and menstrual dates was weak in trisomy 18. Gestational age predicted by crown-rump length was significantly lower than gestational age by menstrual dates. All long bones were significantly shortened and fell below normal regression lines for gestational age. The fetal femur/foot length ratio was reduced. Thus no endogenous measure of gestational age appears to exist in this aneuploidy, forcing reliance on menstrual dates. The observed pattern of growth alterations will likely preclude the development of a gestational age-dependent biometric screen for the prenatal detection of this syndrome.