Secular changes in dizygotic twinning rates

In Australia, Belgium, Denmark, England and Wales, Holland, Italy, New Zealand, Norway, Scotland, Sweden and Switzerland there was a substantial decline in dizygotic twinning rates during the 1960s. In Portugal, Spain and Japan there was a lesser decline and in the US there was little change. In many of the countries for which evidence is available, it seems that these declines all started at about the same time in the late 1950s. It has been suggested that the recent decline in Scotland occurred mainly in middle class women; in contrast it is suggested here that the earlier decline in the US during the years 1933–58 was more characteristic of lower class women. For England and Wales and for Scotland, the recent decline (unlike that in the US 1933–58) occurred about equally in young and older women, so that the causes of the declines in the US in 1933–58 and in other countries at present may be different. In the absence of hard evidence, it is speculated that the current declines may be due to hormonal or pesticidal substances widely used in agriculture. Monozygotic twinning rates have shown no similar trend so that pregnancy wastage does not seem to be implicated.