The Northwestern University Multihospital Twin Study. II. Mortality of first versus second twins.

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.