The prenatal mortality rate (PMR) was analyzed using Japanese Vital Statistics for 1950-1991 in Japan. Changes in the PMRs were analyzed according to sex and single-multiple births. This paper also investigated the effects of maternal age, birth order, gestational age, and birthweight on the PMR.
The results obtained led to the conclusion that the PMR significantly decreased with the year. The PMR was higher in males than females, but the differences between sexes decreased with the year. The ratios of multiple to singleton PMRs were 6-7 times for twins, 12-13 times for triplets, 15-22 times for quadruplets, and 50-87 times for quintuplets during the period 1980-1991. The PMR was higher in the second-born than the first-born twins, higher in the third-born than the second-born triplets, and higher in the second-born than the first-born triplets. But the birth order effects were not seen in quadruplets. The PMR increased with birth order except in the first birth for 1968-1991. As for the birthweight, the PMR was the lowest between 3.0 kg and 3.4 kg for singletons and between 2.5 kg and 2.9 kg for multiples during 1979-1991. For the birthweight less than 2.5 kg, the PMR was higher in singleton than multiples, but this tendency was opposite for the birthweight more than or equal to 2.5 kg. As for birthweight and gestational age, the effect of birthweight on the PMR was more effective than gestational age except shorter gestational age group.