**2. Importance of multiple pregnancy**

In the last years, the rate of multiple pregnancies has raised all over the globe. In the USA, it rose from 18.9in 1980 to 33.4 twins per 1000 births in 2016. The twin birth rates were higher in black women, followed by non-Hispanic white women. The triplet and high-order multiple birth rate has decreased about 48% in the last 8 years, from 193.5 in 1998 to 101.4 twins per 100.000 births in 2016 [7]. This decrease in high-order multiple pregnancies illustrates the reproductive medicine societies' strategies for reducing the risk of high-order pregnancies, like single-embryo transfer and multifetal pregnancy reduction [10–12].

In England, there is also an increase in multiple births. From 1998 to 2016, the multiple maternity rate rose from 14.4 to 15.9 twins per 1000 births. Since 1993, women aged 45 and over have consistently recorded the highest multiple maternity rate. These changes in the multiple pregnancy rates are due to the increase in ART. It is estimated that in vitro fertilization (IVF) conceptions are 11 times more likely to result in a multiple birth than natural conceptions. In 2014, 16% of IVF pregnancies resulted in multiple birth, with nearly 19,000 IVF babies born in the UK in 2014 [13].

This trend was largely attributed to an elevated amount of dizygotic pregnancies, without significant variations in monozygotic births over the past few decades. The dizygotic twinning rate is affected by many factors such as race, previous multiple pregnancy, maternal age and parity, lifestyle, season, use of fertility drugs and treatments, genetics, and others [14–16].

The high number of multiple births impacts directly in rate of preterm birth and low birthweight. Data from 2016 show that among twin pregnancies, 59.9% are born before complete 37 weeks of gestation, while in singletons, only 8% are preterm births. In singleton births, 6.4% were born with weight less than 2500 g. This percentage is 55.4 in twins and more than 95% in triplets [7].
