Why do small twins have a lower mortality rate than small singletons?

Abstract

We propose an interpretation of the paradoxically better survival rate among low-birth-weight twins compared with low-birth-weight singletons. We used data from Belgian birth and death certificates for 1983 and 1984. The data include 229,964 singletons, 2175 first twins, and 2153 second twins. Weight-specific perinatal mortality rates of twins and singletons were compared; the birth-weight distributions were adjusted to a single mean and SD. After adjustment, mortality rates at every weight were higher for twins than for singletons. The appearance of better survival among small twins compared with small singletons disappears after adjustment to relative birth-weight. There is a large risk resulting from twinning that falls on all twins, regardless of their weight.