Over the last decades the frequency of multiple births has been on the decline in most developed countries. This trend, however, has been restricted to dizygotic twins, while monozygotic rates have remained stable or risen slowly. In more recent years, however, the fall in multiple dizygotic birth rates has ceased and a slight increase is observed. This trend and the increased frequency of triplets or higher-order births, registered from the early 1980s onwards, are essentially related to treatments for infertility. No single risk factor, such as maternal age, parity, oral contraceptive use, or declining fertility rates can explain the overall declining trends. In this paper we review the descriptive epidemiology and the main risk factors for multiple pregnancies.