Perinatal mortality in single and multiple births in Japan, 1980-1991.

Perinatal mortality rates (PMR) decreased significantly per year in single, twin, triplet, quadruplet and quintuplet births during the period 1980-1991. The PMRs were 7.7 per 1000 livebirths for singletons, 45.6 for twins, 89.0 for triplets, 116.8 for quadruplets, and 476.2 for quintuplets during the 12 years. The relative risks of perinatal death in multiplets vs. singletons were 6 for twins, 12 for triplets, 15 for quadruplets and 62 for quintuplets. The PMR was significantly higher in males than females for singletons and twins, but there was no sex differential in PMRs for higher order of multiple births. The PMR increased with birth order in twins and triplets, whereas there was no birth order effect on the PMR for quadruplets. An increasing proportion of multiple births among perinatal deaths may be related to the increasing multiple birth rate in Japan.