Author(s): Imaizumi Y, Nonaka K
The stillbirth rates decreased to 2/3 for MZ male twins, 1/2 for MZ female twins, and under 1/2 for DZ twins for both sexes during the 19-year period from 1975 in Japan. The stillbirth rate was significantly higher in MZ males than MZ females in each year, whereas stillbirth rates of DZ twins for both sexes indicated similar values during that period. After 1986, stillbirth rates were more than 2 times higher in MZ twins than in singletons and in DZ twins. The higher stillbirth rate of MZ twins as opposed to DZ twins could be related to monochorionic twin pairs in MZ twins. The stillbirth rate decreased more drastically in twins for both zygosities than in singleton births during the 34-year period from 1960. However, declining rates of stillbirths may be attributed to medical care during twin pregnancies. Recommendation of an optimum day to give birth for twin pregnancy is 37-38 weeks for Japanese women.