Factors influencing the stillbirth rates in single and multiple births in Sweden, 1869 to 1967.

Temporal variations in the stillbirth rate among singletons, twins and triplets in Sweden between 1869 and 1967 were studied. Both among single and multiple births there were marked secular decreasing trends in the stillbirth rates. Based on our long time series since 1869, this study confirms that among twins and triplets the stillbirth rate was higher among same-sexed than among opposite-sexed sets. Comparisons between the stillbirth rates among twin births in urban and rural regions indicate higher stillbirth rates in rural areas. In addition, the stillbirth rates among twins of unmarried mothers were higher than those of twins of married mothers. These findings also hold for both same-sexed and opposite-sexed twin pairs. Analyses of the stillbirth rates for singletons and for different types of twins indicate that up to 1950 the risk of stillbirth among males was almost constantly between 15% to 20% higher than among females. After that the difference in the risk decreased. Comparisons with other populations were performed.