Attempts have been made to identify factors influencing the number of males per 100 females at birth, also called the secondary sex ratio. It has been proposed to vary inversely with the frequency of prenatal losses, but available data lend at best only weak support for this hypothesis. Statistical analyses have shown that comparisons between secondary sex ratios demand large data sets. Variations in the secondary sex ratio that have been reliably identified in family data have mostly been slight and without a notable influence on national birth registers. For Sweden, 1751-1950, the secondary sex ratio among all births and live births revealed increasing trends. The Swedish results are compared with available findings for live births in Finland, Norway, Denmark, and the small Icelandic population. For Norway and Denmark, the secondary sex ratio increased during 1801-1950. A similar, but stronger pattern was observed for Finland (1751-1950) and Iceland (1838-1950). During the latter half of the twentieth century, marked decreases were observed in all countries. Attempts to identify reliable associations between secondary sex ratios and stillbirth rates have been made, but no consistent results have emerged.