WHY DO SMALL TWINS HAVE A LOWER MORTALITY-RATE THAN SMALL SINGLETONS

OBJECTIVE: We propose an interpretation of the paradoxically better survival rate among low-birth-weight twins compared with low-birth-weight singletons. STUDY DESIGN: 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. RESULTS: After adjustment, mortality rates at every weight were higher for twins than for singletons. CONCLUSIONS: 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.