SEX-RATIOS IN FETUSES AND LIVEBORN INFANTS WITH AUTOSOMAL ANEUPLOIDY

Ten data sources were used substantially to increase the available dat a for estimating fetal and livebirth sex ratios for Patau (trisomy 13) , Edwards (trisomy 18), and Down (trisomy 21) syndromes and controls. The fetal sex ratio estimate was 0.88 (N = 584) for trisomy 13, 0.90 ( N = 1702) for trisomy 18, and 1.16 (N = 3154) for trisomy 21. All were significantly different from prenatal controls (1.07). The estimated ratios in prenatal controls were 1.28 (N = 1409) for CVSs and 1.06 (N = 49427) for amnioeenteses, indicating a clear differential selection against males, mostly during the first half of fetal development. By c ontrast, there were no sex ratio differences for any of the trisomies when comparing gestational ages (16 and >16 weeks. The livebirth sex r atio estimate was 0.90 (N = 293) for trisomy 13, 0.63 (N = 497) for tr isomy 18, and 1.15 (N = 6424) for trisomy 21, the latter two being sta tistically different than controls (1.05) (N = 3660707). These ratios for trisomies 13 and 18 were also statistically different than the rat io for trisomy 21, Only in trisomy 18 did the sex ratios in fetuses an d livebirths differ, indicating a prenatal selection against males >16 weeks. No effects of maternal age or race were found on these estimat es for any of the fetal or livebirth trisomies. Sex ratios for translo cations and mosaics were also estimated for these aneuploids. Compared to previous estimates, these results are less extreme, most likely be cause of larger sample sizes and less sample bias. They support the hy pothesis that these trisomy sex ratios are skewed at conception, or be come so during embryonic development through differential intrauterine selection. The estimate for Down syndrome livebirths is also consiste nt with the hypothesis that its higher sex ratio is associated with pa ternal nondisjunction. (C) 1996 Wiley-Liss, Inc.