1. Introduction

Currently, twin pregnancies account approximately for 3% of all live births [1]. The number of twin pregnancies has dramatically increased over the last four decades in all of the countries where we have information from vital statistics [2]. One of the main reasons for this recent increase of twin pregnancies is associated to the fact that mean maternal age at childbearing has considerably raised. The responsible factors are socio-economic contingencies and the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

increase in the female employment rate. There is evidence indicating that the incidence of twin pregnancies is known to rise with mother's age [3]. This pattern has been attributed both to an increase in the level of gonadotropins with the age and to the rise of medically assisted reproductive technologies (ART) among older women.

of this relationship between the mother and the child-to-be may differ among woman who are

Prenatal Attachment in Twin Pregnancy http://dx.doi.org/10.5772/intechopen.79365 83

2.1. Twin pregnancy: fertilization, intrauterine growth and associated fetal risk factors

Twin pregnancy can be the result of multiple ovulations with fertilization of any oocyte by a sperm and in that case we have dizygotic twins (DZ) or a consequence of the fertilization of an oocyte by a sperm with subsequent division of the single zygotes and in this case we will have monozygotic twins (MZ). All DZ pregnancies are dichorionic (two placentas) and diamniotic (two amniotic cavities). MZ pregnancies, in relation to the gestational age in which the division into two embryos occurs, may be dichorionic and diamniotic (about 1/3 of the cases) if the division takes place between the first and the third day of gestation or monochorionic (single placenta) and diamniotic (about 2/3 of cases) if the division takes place between the fourth and the eighth day of gestation. Finally, the division could rarely occur between the ninth and the thirteenth day of gestation, resulting in monochorionic monoamniotic pregnancy (about 1% of the MZ pregnancies). Overall, DZ twins represent 70% of twin pregnancies and MZ twins represent 30% [20, 21]. From the genetic point of view, DZ twins (fraternal twins) can be assimilated to natural brothers, while MZ twins (identical twins) have always been thought to have the same genetic heritage. However, epigenetic alterations and environmental factors may be responsible for different phenotypic expressions at physical, neuropsychological and

In twin pregnancies, it is essential to define if twins share or not placenta as soon as possible. Chorionicity determination in the first trimester is almost 100% accurate [20]. The most reliable signs to determine chorionicity are the number of gestational sacs between 7 and 10 weeks of gestation and the presence of lambda sign (a subtle triangular strip of cortical tissue separating the two placenta) between 11 and 14 weeks of gestation [21, 22]. The determination of chorionicity is particularly important from a clinical and prognostic point of view, since monochorionic (MC) twin pregnancies are complicated by an incidence of 10–15% of twin-to-twin transfusion syndrome (TTTS). TTTS is a chronic midtrimester complication of MC twin pregnancies that causes significantly higher perinatal mortality and morbidity rates in monochorionic than in dichorionic twins [23]. MC twins share their placenta and their blood circulation is connected by vascular anastomoses at the placenta surface. Placenta vascular anastomoses allow acute or chronic inter-twin blood transfusions between the circulations of the two fetuses. Imbalanced inter-twin blood flow can lead to a severe complication such as TTTS. In TTTS, imbalanced blood flow from one twin (the donor) to the other twin (the recipient) results in hypovolemia and oligohydramnios in the donor and hypervolemia and polyhydramnios in the

2. Medical issues associated to twin pregnancy and twin birth

behavioral levels, in the absence of variations in the genetic sequence.

recipient twins with transient or persistent right ventricular hypertrophy [24].

Another important issue related to twin pregnancy concerns fetal growth. In single pregnancies, progressive and linear fetal growth is observed until the 37th week of gestation, whereas

expecting twins and those who are expecting singleton [19].

This extraordinary growth in twinning rates in different developed countries must be considered as an important public health issue since twin pregnancies are generally associated with greater risk for both infants and mothers. Twin babies are more fragile, have lower birth weight and born preterm more often than singleton babies. In addition, many of the risks to the mother are also risks to the child-to-be, since they can lead to premature labor, complications or, in the worst cases, fetal death. For these reasons, twin babies are more frequently admitted to neonatal intensive care units (NICUs) and subjected to more prolonged hospitalization with potential negative effects both on infants and parental behavior. Other complications for the mother are gestational diabetes, hypertension, preeclampsia and acute polyhydramnios.

Moreover, twin births can have negative effects on parents' adjustment as well. In fact, although it is possible to identify similarities in pregnancy and parenthood for twin and singleton births, the experience of expecting and parenting twins seems to be very different [4]. The responses to a multiple pregnancy and parenthood by most of the parents may be associated with ambivalence and surprise, even if the pregnancy resulted from infertility treatment [5], as well as with higher levels of anxiety, distress and higher risk of depression in the postpartum period both in the case of twins conceived naturally and in the case of twins conceived with ART [4, 6]. In addition, due to the medical risk associated, twin pregnancies need to be closely monitored [7, 8]. On one hand, this frequent and intensive monitoring could reassure parents, but, on the other, it constantly reminds them that their pregnancies could be associated with serious risks for the babies and the mothers. Different researches tried to evaluate the association between the presence of mood disorders and stress in parents and twin pregnancies. Researchers that focused on parental experience associated to twin pregnancy that occurs both naturally or with ART investigated either the joint experience of mothers and fathers [9–12] or of mothers alone [13–16]. It was observed that the risks usually associated to twin pregnancy lead to higher level of stress [17] and seemed to increase the incidence of depression and anxiety in parents of twins and especially in mothers [6, 9–11, 16, 18]. In addition, it was also observed that the presence of medical risk as well as psychological suffering in mothers during twin pregnancy is generally associated to higher level of fatigue, loss of energy, depressed mood and feelings of worthlessness and guilt [14].

Another important aspect related to the mothers' psychological adjustment during twin pregnancies is related to the building process of the relationship between the mother and the childto-be [19]. During pregnancy, mothers use to think about their child-to-be, and they start to create representation of themselves as mothers. During twin pregnancies, the mother-to-be has to deal with an identification process with two children at the same time and have to create a mental space that allow her to make representation of both children. These processes include representations of physical and emotional characteristics of two different fetuses and of the interactions between the mother and her future babies, as well as dreams and expectations about both the children-to-be. For these reasons, it is possible to infer that the building process of this relationship between the mother and the child-to-be may differ among woman who are expecting twins and those who are expecting singleton [19].
