Trends of twinning rates were analyzed using vital statistics in Austria, Finland, Norway, Sweden, Canada, Australia, Hong Kong, Israel, Japan, and Singapore during the period from 1972 to 1996. The twinning rates increased significantly year by year in each country. During these periods, the twinning rate increased by twenty percent in Austria and Canada, and by sixty percent in Norway and Sweden. The twinning rate was 1.6 times higher in Sweden than in Hong Kong in 1972 and by eighty percent in Israel in 1995. Twinning rates were higher in European countries, Canada, and Australia than in Asia. The variations of the twinning rates among countries were not only due to biological factors, but also to assisted reproductive techniques. In Australia, the overall twinning rate was 1.3 times higher in the nuptial births (14.1 per 1000 births) than in the ex-nuptial births (10.7) during the period 1994-1996. As for maternal age, twinning rates in Sweden increased year by year for maternal age groups except the youngest and the oldest age groups. In Sweden, the rising twinning rate has been attributed to the higher proportion of mothers (for the 25-39 year old age groups) treated with ovulation-inducing hormones and attributed to in-vitro fertilization.
PIP: This document presents the result of the 1972-96 study on the trends of twin raising in Austria, Finland, Norway, Sweden, Canada, Australia, Hong Kong, Israel, Japan, and Singapore. An overall increase in twinning rate was noted among the 10 countries, although the statistics is higher among Caucasians compared with Asians each year. A 20% increase was noted in Austria and Canada and 60% in Norway and Sweden. In 1972, twinning rate in Sweden is higher by 1.6% compared with Hong Kong and 80% in Israel in 1995. The rise of multiple births can be attributed to the use of ovulation-inducing hormones and other reproductive techniques. Nuptial twin births are 1.3% higher in Austria compared with nonnuptial twin births. An annual increase in the rate of twin births among 20-24 and 35-39 year old groups was noted. The 60% increase in twin birth rate among Swedish mothers can be attributed to a higher proportion of mothers over age 30 treated with ovulation-inducing hormones and in vitro fertilization, which is similar in the countries of England, Wales, Denmark, and Japan.