Perinatal mortality in twins and factors influencing mortality in Japan, 1980--98.

The perinatal mortality rate (PMR) in twins was analysed using Japanese vital statistics during the period 1980-98. There was a total of 1992 twin perinatal deaths. The PMR in twins significantly decreased from 91 per 1000 births in 1980 to 31 per 1000 in 1998. The PMR was higher in males than females, but the differences between the sexes decreased over time. It was significantly higher in the second-born than the first-born of the twins. The fetal death rate was 2.8 times higher in monozygotic than dizygotic twins. The PMR decreased to one-sixth for the maternal age group > or =40 years, one-fifth for <20 years, and between one-third and a half for 20--39 years during the period 1980--98. It was lowest for birthweights (BW) between 2500 g and 2999 g during the entire period; the PMR decreased with gestational age (GA) up to 38--39 weeks and increased thereafter. The effect of BW on the PMR was greater than that of GA except in the shorter GA group. The proportion of twin perinatal deaths with extremely low BW (<1000 g) increased from 50% in 1980 to 68% in 1998. Thus, the declining PMR was unlikely to be due to the improvement in BW in twins. It is likely that it is related to the improved medical management of twins during the perinatal period and the first week of life.