Temporal and regional trends in the secondary sex ratio: The Nordic experience

The sex ratio (SR) at birth, also known as the secondary sex ratio, is defined as the number of males per 100 females and approximates 106. According to the literature, the SR shows notable heterogeneity and attempts have been made to identify factors influencing it, but comparisons demand large data sets. Attempts to identify associations between SRs and stillbirth rates (SBRs) have yielded inconsistent results. A common pattern observed in different countries is that during the first half of the twentieth century, the SR showed increasing trends, but during the second half, the trend decreased. Secular increases are thought to be caused by improved socio-economic conditions. The recent downward trends have been attributed to new reproductive hazards. Similar findings have been made in the Nordic countries. Factors affecting the SR within families remain poorly understood. Although these factors have an effect on family data, they have not been identified in large