**1. Introduction**

The definition of multiple pregnancies describes the presence of more than one fetus in a single pregnancy. Usually the reference to multiple pregnancies refers to twins, which means that two fetuses coexist in the same pregnancy in one uterus. Twin pregnancy results from the fertilization of two ova from a corresponding sperm (dizygotic twins) or from the fertilization of an ovum after separation of the resulting zygote with the final result of the creation of two embryonic monozygotic twins. Each fetus is surrounded by two membranes, one external (well known as chorionic) and one internal (well known as amniotic). Possibly, more than two embryos will be created in multiple pregnancies, but the more there are, the less likely to survive in the future. The frequency of multiple pregnancies in the human species can be calculated based on Hellin's rule and therefore the following data emerge: for twin pregnancy 1: 85 (1.18%), for triple pregnancy 1:85<sup>2</sup> (0.013%) and for quadruple pregnancy 1:853 (0.0061%) [1–4].

The 2/3 of them are polygenic pregnancies and 1/3 are monogenic pregnancies. Regarding twin pregnancies, about 70% are dizygotic and 30% are monozygotic twins. The incidence of multiple pregnancies has generally increased in recent years from 1/100 to 1/60–1/70 births. The exact frequency of twins is very difficult to determine because in the first trimester they have a high frequency of miscarriages, much higher than single pregnancies and compared to that in the third trimester of pregnancy to be doubled. Spontaneous abortion of one twin without affecting the other is often asymptomatic. While the frequency of monogenic pregnancies does not depend on the race, heredity, age of the mother, as well as her ability to fertilize after administration of drugs to induce ovulation, the frequency of polygenesis is influenced by the above factors [1–4]. Twin pregnancies account for 1% to 3.5% of total births and have a 15% perinatal mortality rate. In Greece, the incidence of twin spontaneous pregnancies is about 1% and in women with IVF is about 25%. The percentage of perinatal mortality in the US has been declining from 12.6% in 1980 to 6.8% in 2001, a decrease of 46%. According to the literature, about 50% of twin pregnancies are associated with prematurity and low birth weight. Compared to single pregnancies, twin pregnancies have four times the risk of perinatal mortality, while in triple and upper multiple pregnancies there is up to nine times the similar risk. Especially in adolescent twin pregnancies and twin pregnancies in women over 35 years are 46.28% and 16.68% respectively [1–8].
