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Lower rates of certain birth defects (e.g., neural tube defects, oral clefts, hypospadias, some types of clubfoot) have been reported in Blacks vs. Whites in various countries. The explanation is unknown, but an hypothesis is advanced regarding maturation rates during critical periods of embryonic-fetal development. Analysis of data from several studies in the U.S. indicates that lower rates among Black relative to White live births for some defects (certain neural tube defects, oral clefts, hypospadias) hold mainly within lower birth-weight categories; different etiological factors may be involved in defects occurring in different birth-weight groups. Additional data needed to explore this hypothesis of accelerated maturation as a factor in reducing the risk of certain defects among Blacks include: (1) malformation rates, from unbiased sources, in Blacks and Whites within categories of gestation length and birth weight, among live births and fetal deaths; and (2) ultrasound data on the development of living embryos and fetuses, and morphologic data on early abortions, by racial group.