Recent trends in the incidence of multiple births and associated mortality in England and Wales.

AIM: To review trends in multiple births and associated mortality in England and Wales since 1975, in the light of trends for earlier years; to assess, within the limitations of the available data, the extent of any association between multiple birth rates and assisted conception and drugs used for subfertility.
METHODS: Data collected routinely in England and Wales, between 1975 and 1994, were retrospectively reviewed. These comprised Office of Population Censuses and Surveys (OPCS) data collected at the registration of live and stillbirths and deaths occurring under the age of 1 year in England and Wales from 1975 onwards. Department of Health data about NHS prescriptions dispensed outside hospitals within England for drugs which could be used in the medical management of subfertility were also included.
RESULTS: The proportion of pregnancies that resulted in a registered multiple birth increased from a low of 9.9 per thousand in 1975 to 13.6 per thousand in 1994. Up to 1992, the increase was seen in all age groups except for women under 20, with the most substantial increase being in women aged 35 to 39. The rise in the rate of triplet and other higher order births was much steeper than that for all multiple births, increasing from 0.13 sets of triplets per thousand maternities in 1975, to 0.41 in 1994. Prescriptions dispensed for selected drugs that may be used for the medical management of subfertility and assisted conception became more common over this period. The fragmented nature of the data precluded direct comparison with changes in multiple birth rates, however.
CONCLUSIONS: Although the causes of the rise since 1980 in multiple birth rates 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.