POPULATION-BASED STUDY OF RATES OF MULTIPLE PREGNANCIES IN DENMARK, 1980-94

Objective: To study trends in multiple pregnancies not explained by ch anges in maternal age and parity patterns. Design: Trends in populatio n based figures for multiple pregnancies in Denmark studied from compl ete national records on parity history and vital status. Population: 4 97 979 Danish women and 803 019 pregnancies, 1980-94. Main outcome mea sures: National rates of multiple pregnancies, infant mortality, and s tillbirths controlled for maternal age and parity. Special emphasis on primiparous women greater than or equal to 30 years of age, who are m ost likely to undergo fertility treatment. Results: The national incid ence of multiple pregnancies increased 1.7-fold during 1980-94, the in crease primarily in 1989-94 and almost exclusively in primiparous wome n aged greater than or equal to 30 years, for whom the adjusted popula tion based twinning rate increased 2.7-fold and the triplet rate 9.1-f old. During 1989-94, the adjusted yearly increase in multiple pregnanc ies for these women was 19% (95% confidence interval 16% to 21%) and i n dizygotic twin pregnancies 25% (21% to 28%). The proportion of multi ple births among infant deaths in primiparous women greater than or eq ual to 30 years increased from 11.5% to 26.9% during the study period. The total infant mortality, however, did not increase for these women because of a simultaneous significant decrease in infant mortality am ong singletons. Conclusions: A relatively small group of women has dra stically changed the overall national rates of multiple pregnancies. T he introduction of ne-cy treatments to enhance fertility has probably caused these changes and has also affected the otherwise decreasing tr end in infant mortality. Consequently, the resources, both economical and otherwise, associated with these treatments go well beyond those i nvested in specific fertility enhancing treatments.