The sex ratio in each developmental week was analyzed in 551 conceptuses from induced abortions for medical or social reasons in Southwest Finland. The ovulation age of the conceptuses varied from five to 24 weeks. There was a clear decrease in the sex ratios with increasing duration of pregnancy. The embryonic sex ratio was 164, the fetal ratio was 111, and the total ratio was 117. According to the regression analysis, the sex ratios at the ages of eight and 14 weeks were 119 and 105, respectively. The results suggest that higher mortality of male versus female conceptuses is restricted to the period of organogenesis. Seasonal or monthly variation in the sex ratio was not found in the present study.
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