There is increasing interest in the outcomes of multiple pregnancies as their numbers rise, mainly owing to advances in fertility-enhancing techniques. In addition, the numbers of multiple births surviving the perinatal period is increasing with the increasing survival of very tiny babies. In order to investigate these outcomes or to evaluate procedures that may improve them, it is important to consider a number of methodological issues that affect the comparability of data both between and within populations. How a birth and a multiple birth are defined, data sources, whether multiple pregnancies or individual births are being counted and the identification of multiple gestations by zygosity and chorionicity will all affect the reported outcome rates. In light of this, perinatal mortality and neurodevelopmental disabilities are examined as adverse outcomes of multiple pregnancies.