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The overall fertility of first marriages was characterized by the time intervals between marriage and the first five births. Each marriage was given a coefficient of parental consanguinity which varied from 0 to .0625. The analysis of the resulting distributions was conducted using the biometrical tool of skewness and kurtosis. This yielded two fertility factors, lactation amenorrhea and intrauterine mortality. We concluded that both factors decrease with increasing parental consanguinity, amenorrhea being the most important component in the fertility increase. The hypothetical explanation is that parental consanguinity decreases the frequency of maternal-fetal incompatibilities and, as the isolate evolves, favors women with genetically short periods of amenorrhea.