Temporal variation in rates of multiple maternities in Sweden (1751-2000)

In this study we have observed strong temporal fluctuations in the rates of multiple maternities in Sweden, 1751-2000. The rates were the highest noted for a whole nation during the last three decades of the 18th century. After the 1930s, there was a marked decrease in the twinning rate. After the extreme minima for the multiple maternities in the 1960s and 1970s, steep increases in the rates are observed. These increases are caused by the introduction of artificial reproduction techniques.
Standardisation of the twinning rate for Sweden partly reduced the temporal fluctuations, but the general pattern remains. Hence, the strong variations are not related to maternal age or parity. The fluctuations in the triplet rate shows similar pattern as the twinning rate.
Following the observation that maternal age and parity cannot satisfactorily explain the variation in the twinning rate, our hypothesis is that the strong demographic and socio-economic changes in Sweden during the second half of the 19th century and the first half of the 20th century, with increasing urbanization, are the main cause of the decreased rates in multiple maternities. The twinning rate being consistently lower in urban than rural regions supports this finding. In addition, the effect of increasing population density and consequently, the disruption of genetic isolates, maintain these observations.