Changes in twinning rate: Italy 1950-1996

In the wide-ranging debate about the potential monitors of population fertility, twinning rate has been considered a candidate. In the developed countries, a decline in spontaneous twinning rate began around 1950 and continued until the late 1970s. The decrease in mean maternal age at delivery and the number of children per family have been considered as possible reasons for the decrease. The rise in twinning rates, which began in the 1980s, is probably due to the spread of assisted reproduction, at least for older mothers.

The temporal trend in twinning rate and the relationship with maternal age at delivery and birth order in the Italian population were analysed.

The recent increase in twinning rate can be mainly attributed to the recourse to assisted reproduction techniques, as demonstrated by the twinning frequency among older primiparae. The twinning rate recently observed in young mothers approximates to the value of the 1950s, which is assumed to represent a 'natural' fertility condition.

On the assumption that twinning is an indicator of fertility, and hence of population fitness, the present data seem to suggest that natural fertility has been slightly increasing in the Italian population.