Abstract: In Japan, the overall incidence rates of birth defects among fetal deaths were 2.5% for singletons and 3.2% for twins during 1979–1985. The difference is significant at the 5% level. The incidence rate of anencephalus among fetal deaths during the period was higher in singletons than twins in each year, whereas the incidence rate of congenital hydrocephalus among twins was similar to that among singletons except in 1985. The concordance rates were 5.7% for anencephalus, 9% for spina bifida, and 15% for congenital hydrocephalus during the period. In two cases one twin had anencephalus and the other congenital hydrocephalus, and in one case one twin had anencephalus and the other spina bifida. Nationwide data on 112 sets of conjoined twins who died as fetuses or in the postnatal period during 1979–1985 were analysed. Female conjoined twins accounted for 60% of cases. The incidence rate of conjoined twins remained constant throughout the period except in 1985. The overall incidence rate was 10 per million births. A maternal age effect was found in mothers over the age of 40, where the highest incidence rate occurred. The incidence rate increased with birth order.