Birth-weight-specific perinatal mortality in Japan, 1989-1993: singleton versus multifetal pregnancies

To investigate the relation between birth weight and perinatal mortality in multifetal pregnancies, which is more than 5 times higher than for singleton infants.

We assessed the incidence of perinatal deaths based on birth weight in 89,566 infants of multifetal pregnancies and 6,025,199 infants of singleton pregnancies in Japan. Perinatal death was defined as stillbirth and early neonatal death (death <1 week of age).

The incidence of perinatal death was consistently lower for infants of multifetal pregnancies than for infants of singleton pregnancies at birth weights of 500-2,499 g. However, the incidence of infants of multifetal pregnancies with birth weights >/=2,500 g consistently exceeded that in singleton infants weighing >/=2,500 g. The relative risk (95% CI) of perinatal death among infants of multifetal pregnancies compared with singleton infants in the same category of birth weight was 0.88 (0.84-0.93) at 500 g. The relative risk decreased to 0.31 (0.25-0.39) at 1,900 g, increased to >1.0 at 2,500 g, to 3.7 (2.2-6.1) at 3,000 g, and to 14.9 (7.8-28.4) at >/=3,500 g.

Japanese infants of multifetal pregnancies reaching >/=2,500 g in body weight have a significantly higher risk of perinatal death than singleton infants in the same category of body weight. Increased monitoring of fetuses of multifetal pregnancies who weigh >/=2,500 g may be helpful in reducing the incidence of perinatal mortality.