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Twin pregnancies account for 2 to 4 percent of total live births, with variable prevalence worldwide. The rate of twin gestation and higher-order multiples has dramatically increased over the last four decades in nearly all countries. In the United States, for example, the rate rose 76 percent from 1980 to 2009 (from 18.9 to 33.2 per 1000), was generally stable from 2009 through 2012, and then rose for 2013 and 2014; the 2014 rate of 33.9 was the highest ever reported. At the same time, a rise in twinning rate was also evident among other developed countries such as France, Germany, Japan, and South Korea. Of all countries, the highest rates were reported in Nigeria and the lowest in Japan. The significance of these trends in population statistics lies in the potential to negatively influence both perinatal morbidity and mortality rates. Fortunately, these demographics appear to be changing for the better with triplets and higher-order multiple births demonstrating a sharp decline of 52 percent from the 1998 peak (193.5 per 1000 live births). More recently, this declining trend is apparent in twin births as well. In 2018, a twin birth rate of 32.6 twins per 1000 births was noted, a 2 percent decline from the 2017 rate of 33.3 per 1000. A similar decline was also observed in the United Kingdom. This is undoubtedly related to improved control of reproductive cycles and advances in assistive reproductive technology (ART) techniques along with the increasing use of elective single embryo transfer. Although the precise etiology of multiple pregnancy is unknown in most cases, the rise in twinning rate has been attributed to the growing and liberal use of fertility-promoting treatments and technology and to delayed childbearing, implying an effect of advancing maternal age. Data from 2013 on ART showed that twin birth rates declined, with IVF accounting for no more than 16 percent. In contrast, methods other than IVF accounted for 20 percent of the twin birth rate. These methods include ovulation induction and intrauterine insemination (IUI) with superovulation. Variations are evident due to maternal age and ethnicity. In 2006, 20 percent of births to women 45 to 54 years were twins compared to 2 percent of births to women 20 to 24 years old.

The high twin birth rates carry a considerable resource implication on both the health care system and perinatal outcome. It has been estimated that the annual cost of caring for multiples born prematurely after ART is in excess of $1 billion annually or $52,000 per infant. Perinatal morbidity and mortality figures among multiples are also sobering. Population-based studies have reported stillbirth and neonatal mortality rates in twins in the order of 18 per 1000 births and 23 per 1000 birth, respectively, compared to 5 per 1000 births in singleton pregnancies (Table 29-1). Survival rates among multiples are also not uniformly distributed, and the effect of chorionicity on survival is profound (Table 29-2...

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