Fellman Johan, Eriksson Aldur W
Folkhälsan Institute of Genetics, Department of Genetic Epidemiology, Helsinki, Finland.
Twin Res Hum Genet. 2010 Feb;13(1):101-8. doi: 10.1375/twin.13.1.101.
Attempts have been made to identify factors influencing the sex ratio at birth (number of males per 100 females). Statistical analyses have shown that comparisons between sex ratios demand large data sets. The secondary sex ratio has been believed to vary inversely with the frequency of prenatal losses. This hypothesis suggests that the ratio is highest among singletons, medium among twins and lowest among triplets. Birth data in Sweden for the period 1869-2004 showed that among live births the secondary sex ratio was on average 105.9 among singletons, 103.2 among twins and 99.1 among triplets. The secondary sex ratio among stillbirths for both singletons and twins started at a high level, around 130, in the 1860s, but approached live birth values in the 1990s. This trend is associated with the decrease and convergence of stillbirth rates among males and females. For detailed studies, we considered data for Sweden in 1869-1878 and in 1901-1967. Marital status or place of residence (urban or rural) had no marked influence on the secondary sex ratio among twins. For triplets, the sex ratio showed large random fluctuations and was on average low. During the period 1901-1967, 20 quadruplet, two quintuplet and one sextuplet set were registered. The sex ratio was low, around 92.0.
人们已尝试确定影响出生性别比（每100名女性对应的男性数量）的因素。统计分析表明，比较不同的性别比需要大量数据集。人们一直认为，第二性别比与产前损失的频率呈反比。这一假设表明，该比例在单胎中最高，在双胞胎中居中，在三胞胎中最低。瑞典1869年至2004年期间的出生数据显示，在活产中，单胎的第二性别比平均为105.9，双胞胎为103.2，三胞胎为99.1。单胎和双胞胎死产的第二性别比在19世纪60年代开始时处于较高水平，约为130，但在20世纪90年代接近活产值。这一趋势与男性和女性死产率的下降及趋同有关。为进行详细研究，我们考虑了瑞典1869年至1878年以及1901年至1967年的数据。婚姻状况或居住地点（城市或农村）对双胞胎的第二性别比没有显著影响。对于三胞胎，性别比呈现出较大的随机波动，且平均较低。在1901年至1967年期间，登记了20组四胞胎、2组五胞胎和1组六胞胎。性别比很低，约为92.0。
