**1.1 Epidemiology**

The frequency of monozygotic twins is 3.5/1000 to 4/1000 births or 0.4% of pregnancies completely stable regardless of the mother's age, race and heredity is determined by the analysis of the morphology of the placenta or genotype. The dizygotic twins affected because of follicular growth by levels of follicle stimulating hormone FSH, and luteinizing hormone LH affected by racial hereditary factors. The fluctuation rates of the twin pregnancies considered to be due in dizygotic twins, since monozygotic twins have stable effect 3.5/1000 to 4/1000 births. The frequency of dizygotic twins is associated with multiple ovulation heredity and shows high geographical variation. The incidence of spontaneous twin pregnancies varies around the world, with rates ranging from 8/1000 to 17/1000 births. In particular, it ranges from 57/1000 in Nigeria, 12/1000 in the United Kingdom, 12.4/1000 in Scotland, 4/1000 in Japan. In particular, in the USA there was an increase of 65% from 68339 in 1980 to 125134 in 2002. In particular the twins have a fivefold increased incidence in various parts of Africa as a whole of 1/50 or 16/1000 pregnancies, due to high levels of gonadotropins, about half the incidence in Asia 1/150 averaged 3/1000 pregnancies in particular in the yellow race Japanese 2.4%. The incidence of twin pregnancies is in the white race 1/90 or 8/1000 pregnancies [9–12].

Twin pregnancies are more common in older mothers. There is a positive correlation between increased maternal age and increased FSH secretion. There is an approximately 2% increase in women over 35 years old due to increased FSH levels after 37 years (twin decrease and increase in miscarriages in high interest rates) 2% increase after the fourth pregnancy, regardless of age and frequency of

#### *Twin Pregnancies Labour Modus and Timing DOI: http://dx.doi.org/10.5772/intechopen.95982*

contact. There is an increase from adolescence to 39 years per year 0.8% per subsequent pregnancy and then a sharp decline, increased rate of uterine anomalies (1:20) [12–14]. The rate of spontaneous conception is increased due to increased FSH and LH during the summer months with a corresponding reduction in winter. Worldwide, the incidence of twin pregnancies is 1/80, which may be due to different levels of gonadotropins. Women with dizygotic twins have statistically significantly increased serum FSH levels during the early follicular phase, as well as an increased rate of FSH release during impulses. With regard to twin pregnancies, there is a direct correlation with the mother's ethnicity, while on the other hand, the father's ethnicity does not affect. In addition, the probability of giving birth to twins is affected by the heredity of the parents, at a rate of 2% on the part of the mother and 0.8% on the part of the father. The various toxic substances present in water and food are also dynamic factors that play a role in the frequency of dizygotic twins. Regarding this fact, in 1988 there was an increase of twins from 3‰ to 20‰ in areas contaminated with polychlorinated hydrocarbons. Finally, in cases of twin pregnancies after in vitro fertilization, 85% of them are dizygotic and 15% monozygotic. The incidence of twins in developed countries is higher than normal due to assisted reproduction methods (IVF, induction of ovulation)) [12–14]. The incidence of twin pregnancies after widespread use of assisted reproduction has increased significantly. From 1/80 pregnancies nearly the frequency has been tripling. Particular the percentage of twin pregnancies using the "in vitro fertilization" (IVF) or" intra cytoplasmic sperm injection" (ICSI) is 18–53%, while with the use of insemination and of controlled ovarian overstimulation (IUI - COH) with induction of ovulation is 6.8% to 11.7% [10–18]. Most twins resulting from assisted reproduction are dizygotic. Significant is the increase in the frequency of twins from 10/1000 births in 1980 to an average of 20/1000 births with the help of assisted reproduction methods. A total of 25% of pregnancy and 1/64 in the United Kingdom are the result of assisted reproduction methods) [14–18]. The incidence of assisted reproductive twins can be significantly reduced depending on the use of the embryo selection method and the selective transfer of an embryo. However, the incidence of monozygotic twin pregnancies after assisted reproduction is higher compared to spontaneous monozygotic twin pregnancies. The incidence of monozygotic twin pregnancies due to assisted reproduction is 0.9%. According to bibliographic sources, blastocyst transfer and maternal age (under 35 years) are associated with an increased risk of both either monozygotic or monochorionic twins after IVF [10–22]. According to literature reports the risk of miscarriage in the first trimester is 43% in twins after spontaneous conception, 51% after assisted reproduction while the respective risk rates of miscarriages are 19% and 21% [10–22]. Twin pregnancy is the result of fertilization of 2 eggs from 2 sperm (dizygotic twins 1.2% of pregnancies) or the division of a fertilized egg by the separation of the zygote into 2 separate entities, forming two embryos that can develop individually (monozygotic twins: 0.4% of births or 4/1000 births) [10–22]. The incidence of monochorionicity due to IVF is 3.2% [10–22]. The percentage of monozygotic twins is stable, in contrast the percentage of dizygotic twins depends on race, various genetic factors, mother's age, endogenous gonadotropins, sex fetus and the use of assisted reproduction methods [10–22].
