Abstract
The trends in British twin and higher multiple birth rates inside and outside marriage from 1938 to 1989 were reviewed for evidence of the probable impact of the use of ovulatory stimulants. After adjustment for maternal age monozygotic twin rates within marriage rose to a significant excess (of 12%) over those outside during the last two decades; a similar increase of higher multiple births within marriage to a significant excess (of 50%) over those outside was also seen for 1980-9. The dizygotic twin rate within marriage exceeded that outside in 1989 for only the second time in 50 years, as the difference in rates narrowed in the last decade. It is concluded that ovulation induction and techniques of assisted conception are apparently more available to the married, with a stronger effect on higher multiple birth rates than on dizygotic twin rates. Monozygotic twin trends support the suggestion that part of the infertility treatment regimen also promotes zygote splitting.