The results of a multihospital study involving a total of 588 twin pairs born in Chicago in 1970–1975 are reported, with special respect to differences in mortality between first and second twins by time as well as by cause of death. Mortality was higher in second than in first twins and most commonly occurred after delivery and was the result of immaturity and of respiratory distress syndrome.

Birth weight data for 356 pairs of newborn twins of known zygosity and placentation are presented. The combined weights of dizygotic twins are heavier than those of monozygotic twins. The significance of this finding is discussed in relation to the association between increased maternal height and dizygotic twinning.

Preliminary observations fail to indicate a higher risk to the twin babies as a result of labor induction or Cesarian section, so that a systematic clinical trial will now be possible.

A study of perinatal mortality in twins has been carried out concurrently in Ibadan, Nigeria, and Aberdeen, Scotland. In both areas perinatal mortality rates have been found to be four times higher in twins than in singletons, reaching up to 310 per 1000 in the monochorionic group in Ibadan. Moreover, rates have been found to increase considerably with maternal age and parity.

An obstetric-neonatal-gynecological data bank has been developed for the Aberdeen District in Scotland and is now fully computerized. Within it, a twin register including over 550 pairs of known placentation and zygosity has developed.

The European Economic Community (EEC) is promoting a network of locally funded centers in the nine EEC countries, surveying a total of approximately 140,800 births per year for selected congenital malformations and multiple births. The objectives and aims of the study are explained and some of the methods to set guidelines detailed.