Editor—Dunn and Macfarlane1 reviewed the possible causes of the secular increase in multiple birth rates in England and Wales since 1980. They concluded: “Although the causes of the rise since 1980 in multiple births in general, and the dramatic rise in the triplet rate in particular, cannot be ascertained or quantified directly from routinely collected data, drugs used for subfertility and more recently, assisted conception, probably had a major role.”

There can be no doubt that these authors are correct. But it is interesting to consider whether the rise is solely iatrogenic. There are reasons for suspecting that the “natural” twinning rate (which had been declining throughout the 1960s and early 1970s in England and Wales) had halted by 1980 and started to increase thereafter.

Dunn and Macfarlane wrote: “Up to 1992, the increase was seen in all age groups except for women aged under 20.” This is not entirely correct: admittedly, the increase was small in that age group, but table 1 shows that an increase has …