OBJECTIVE: To describe the trends in multiple maternity rates and the main responsible factors, and to present the main medical risks associated with multiple births. METHOD: A review of studies describing the situation in Europe and America North. RESULTS: Multiple maternity rates have increased dramatically in France and in other Western countries since the mid 1970s. However the triplet maternity rates leveled off in the 1990s in several European countries. One-fourth to one-third of the increase in multiple maternities is explained by the increase in maternal age. Moreover 30 to 50% of twin maternities and at least three-fourths of triplet maternities would occur after fertility treatments, according to surveys or estimations made from various sources. Twins and triplets have high risks for mortality and morbidity. In France in 1995/98, among live birth twins, the proportion of births before 37 weeks was 43.7% and the proportion of births before 33 weeks was 8.1%. The risk fraction of preterm delivery (<37 weeks) attributable to twins rose from 13% in 1981 to 19% in 1995/98. This trend is explained by the increase in occurrence of multiple births as much as by an increase in preterm births among twins. A similar situation is found in Canada and the USA. Whereas triplets have very high risk of preterm birth, the risk fraction attributable to triplets is very much lower than for twins: in Canada it was 1.1% in 1995-97. CONCLUSION: These results suggest that research and preventive interventions should not be restricted to triplets, but should also include twins.