Among their important conclusions are a) the birth rate of twins and other multiples (particularly triplets and higher) are on the rise, the perinatal mortality rate for multiples is higher than for singletons (and higher for monozygotic "identical" twins than dizygotic "fraternal" twins), and the long-term consequences of a pregnancy of multiples can be both numerous and costly.

As has been reported by many experts, the increasing incidence of multiple births in the United States is due to the trend toward delaying child birth and the increasing use of assisted reproductive technologies. These changes have primarily occurred since the onset of the new "baby boom" which began in the mid-1970's. The number of twin births paralleled the number of singleton births as well as the number of triplet births and all remained fairly constant prior to 1975. Then, dramatic changes occurred in the rates of these births between 1973 and 1990. Twin births increased at twice the rate of singletons (65 versus 32%) and triplet and higher order births increased at 7 times the rate of singletons (221 versus 32%).

According to Professor Charles Bocklage of the East Carolina University School of Medicine, one out of every eight human conceptions begins as twins but that for every twin pair born alive, 10-12 twin pregnancies result in a singleton birth.

One interesting result of the study of twin research is that some cultures there exists an unexplained seasonality of twinning. In Japan for instance (a nation which has among the lowest percentage of twins of any country), there was a dramatic increase in twin births (both identical and fraternal) during the months of July, August and October.

Several studies summarized online show the dangers of multiple births. One 1988 study reported that 11.2 of every 1,000 births of singletons in the United States resulted in infant death (birth to one year) while 66.4 of every 1,000 births of twins resulted in infant death and 190.4 of every 1,000 births of triplets resulted in infant death.