Are 'natural' twinning rates continuing to decline?

During the 1960s and 1970s, the maternal age-specific twinning rates were declining in England and Wales and many other countries. However, around 1980 this change reversed and since then twinning rates have been increasing. The question arises whether this increase is simply a consequence of hormonal induction of ovulation and various other techniques of assisted reproduction. The question assumes some importance because the level of "natural' twinning may be an index of reproductive (and other) health. Nationwide data on the prevalence of medically assisted conceptions are not available, so indirect methods have to be used to answer the question. It is tentatively concluded here that the "natural' twinning rate has recently been increasing in England and Wales and Belgium. These conclusions are based on two assumptions: (i) that in England and Wales, conceptions by young women are infrequently preceded by medical assistance; and (ii) in Belgium, the rate of medically assisted conceptions is not lower in East Flanders than in the rest of Belgium. Direct data on these points would test these conclusions.