The epidemiology

The epidemiology of twin–twin transfusion syndrome

Multiple births carry a high risk of antenatal and delivery complications to both mother and child.1,2 These complications reflect directly and indirectly on additional social and economic costs borne by both the families and the respective healthcare system.3,4 With the rising rates of twins and higher-order births worldwide, it is becoming more necessary to direct healthcare attention to challenges we have and will continue to face. Twins are the largest proportion of multiple births, constituting 94–98.5% of all multiple births.

The average twin rate in North America is estimated to be 9.6–26.8/1000 births (Figure 2.1a, b). Rates as high as 57.2/1000 births have been reported in Africa,7 whereas Japan has quoted one of the lowest rates in the world at 4/1000 births.8 The rates of twin pregnancies in the United States have shown a 42% increase between 1980 and 19975 and a Canadian survey demonstrated a 35% increase between 1974 and 1990.6 These increasing rates are believed to reflect the advancement in reproductive technology as well as advancing maternal age.


The rise in twin rates may partly be explained by the relatively lower rate of singleton births despite the expanding population.5 Recently, a significant proportion of deliveries following assisted reproductive technology have been multiple pregnancies (Table 2.1). In the majority of surveys, the rate of twins expressed per thousand births will obviously exclude the large numbers of multiple conceptions that are inadvertently lost prior to 20 weeks. In one survey early-stage pregnancy loss has accounted for 15–20% of conceptions.

Monozygotic twinning occurs in 0.42% of all births.9 Two-thirds of all monozygotic twin gestations are of monochorionic placentation. Placentation rather than zygosity is clinically relevant because of the increased risk of mortality and morbidity associated with monochorionicity. 10,11 Recently, monochorionicity in dizygous twin pregnancies has been reported.12–15 Monozygotic twinning appears to be increased after assisted reproductive technologies, including ovulation induction and in vitro fertilization,16 and may be associated with blastocyst transfer compared with day 3 embryo transfer.9