Imaizumi Y, Nonaka K
National Institute of Population and Social Security Research, Ministry of Health and Welfare, Tokyo, Japan.
Acta Genet Med Gemellol (Roma). 1997;46(2):87-98. doi: 10.1017/s0001566000000635.
Zygosity of triplet births in Japan was estimated by Allen's equation with the assumption that the rate of dizygotic (DZ) triplets reflects that of twins. Whereas the DZ triplet rate increased during the period from 1975 to 1994, reflecting the increase in the DZ twinning rate in the period, monozygotic (MZ) triplet rates remained constant from 1975 (28 per million births) to 1994 (23). The trizygotic (TZ) triplet rate gradually increased from 1975 (18) up to 1985 (29), and rapidly increased thereafter to 1994 (202). The higher TZ triplet rate since 1986 is most likely attributed to the higher proportion of mothers treated with ovulation-inducing hormones and partially to in-vitro fertilization in Japan. As for maternal age, MZ triplet rates remained nearly constant for all the maternal age groups except the youngest and the oldest ones. On the other hand, TZ triplet rates increased up to the age group of 30-34 years and decreased thereafter in almost every year. The TZ rate in the age group of 30-34 years slowly increased from 1975 to 1988 (63 per million births) and rapidly increased thereafter (314 in 1994). The TZ rate was statistically significantly higher in the period 1986-1994 than in the period 1975-1985 in each of the nine districts in Japan. Geographical variations in the TZ rates in the latest period have drastically changed from those during the period from 1955-1959 and in 1974.
在日本，三胞胎出生的合子性是通过艾伦公式估算的，假设双合子（DZ）三胞胎的发生率反映了双胞胎的发生率。在1975年至1994年期间，DZ三胞胎发生率有所上升，反映出该时期DZ双胞胎发生率的增加，而单合子（MZ）三胞胎发生率从1975年（每百万例出生28例）到1994年（23例）保持不变。三合子（TZ）三胞胎发生率从1975年（18例）逐渐上升至1985年（29例），此后迅速上升至1994年（202例）。1986年以来较高的TZ三胞胎发生率很可能归因于接受促排卵激素治疗的母亲比例较高，部分原因是日本的体外受精。至于产妇年龄，除最年轻和最年长的年龄组外，所有产妇年龄组的MZ三胞胎发生率几乎保持不变。另一方面，TZ三胞胎发生率在30 - 34岁年龄组之前上升，此后几乎每年下降。30 - 34岁年龄组的TZ发生率从1975年到1988年缓慢上升（每百万例出生63例），此后迅速上升（1994年为314例）。在日本的九个地区中，1986 - 1994年期间的TZ发生率在统计学上显著高于1975 - 1985年期间。最新时期TZ发生率的地理差异与1955 - 1959年期间和1974年相比发生了巨大变化。
