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Previous studies have reported regression formulae describing organ weight changes in often circumscribed time periods of human prenatal development. This study reports regression formulae characterizing growth in ten selected fetal organs relative to changing organ weight in 80 singleton abortuses assessed as being morphologically normal or typical-for-age, and representing weeks 13-31 (fertilization age) of pregnancy. Correlation analyses suggest that total fetal body weight or brain weight may be better reference parameters against which to assess increasing organ weights during much of the fetal period. Fetal body weight rather than the commonly used crown-rump length or menstrual age is the best reference parameter for assessing either growth of a single organ or in the generation of regression formulae. Of the ten organs monitored, the analyses of this study indicate that changes in brain weight are most highly correlated with changes in either crown-rump length or total fetal body weight. Organ weight correlations become of particular significance in the understanding of defect clusters involving organs seen in birth defect syndromes.