Infant mortality rates in single, twin and triplet births, and influencingfactors in Japan, 1995-98

The infant mortality rate (IMR) was analysed among single, twin and triplet births during the period from 1995 to 1998 using Japanese Vital Statistics . This study also investigated the effects of order of multiple births and of birthweight on the IMR. Proportions of neonatal deaths among total infan t deaths were about 1/2 for singletons and 3/4 for both twins and triplets. Thus, to reduce the FAR, intensive care of multiple births is likely to be very important during the first month of life. The IMR was higher in males than females for both singletons and twins, but not in triplets. Relative risks of the IMR in multiples relative to singletons were 5-fold in twins a nd 12-fold in triplets. The IMR was higher in the second-born (18 per 1000 live births) than the first-born (16) twin and higher in the third-born (51 ) than the first-born (31) and the second-born (34) triplet. The higher ris k in the second-born than the first-born twin may be related to delivery co mplications. The IMR decreased rapidly as birthweight increased in singleto ns, twins, and triplets. IMRs for less than or equal to 1500 g were: 2.4 pe r 1000 live births in singletons, 5.9 in twins and 6.1 in triplets. The cor responding proportions of infant deaths were 75%, 33% and 10% respectively. The higher relative risks of multiple births are almost entirely the resul t of the lower birthweight distribution among twins and triplets. To reduce the IMR, birthweight is an important factor in twins, triplets and singlet ons. The overall early neonatal death rate decreased as gestational age ros e in singletons, twins and triplets. For birthweights <1000 g, higher IMRs were related to gestational ages of <28 weeks.