**11. Monochorionic twins discordant for fetal defects**

The rate of congenital anomalies in twins is 2 times higher than in singletons [102, 103]. In monozygotic twins it is around 5-fold greater. However, in dizygotic twins this rate is similar to singletons. One of the main causes of congenital anomalies in monochorionic twins is related to vascular disruption. The past concept that all monozygotic twins are always identical has changed. The rate of concordant congenital anomalies is 9-18%, even in monozygotic twins [104]. Actually, monozygotic twins are rarely identical once genetic differences exist [105].

There are specific anomalies related to multiple pregnancies, explained by the twinning process and aspects of placentation. The abnormalities of monozygotic twinning include: conjoined twins, TRAP sequence, parasitic twins, and fetus-in-fetu [106]. Monochorionic twin pregnancies have placental vascular anastomoses that could result in TTTS in 15% of cases [107]. Congenital heart defects is 3-fold increased in monochorionic pregnancies with TTTS predominantly affecting the recipient twin, such as ventricular septal defects, pulmonary stenosis and atrial septal defects [108, 109]. The rate of fetal anomaly in monoamniotic pregnancies is around 25%, even if conjoined twins are excluded [106].

A discordant fetal defect in a dizygotic twin pregnancy is easy to explain, since the genetic material is distinct. However, in monozygotic pregnancies, discordant congenital anomalies are related to several mechanisms: missegregation of cytoplasmic material (resulting in different characteristics due to post-zygotic mitotic crossing over or non-disjunction), inacti‐ vation or expression of selected genes, imprinting and telomere size differences, X-inactivation and discordant cytoplasmic segregation [110, 111]

Whenever there is a post zygotic non-disjunction in one of the twins, there might be an eventual chromosomal aneuploidy discordance related to chromosomal mosaicism in various degrees. Thus, monozygotic 46, XY and 46, XX twins may be a product of a 46, XXY zygote**.** Single gene mutation discordances involving either nuclear or mitochondrial DNA as well as X-inactiva‐ tion and imprinting discordances have occurred. Environmental factors play a major role in epigenetic differences, considering its greatest impact lays on monozygotic twins who were apart the longest [105].

In monochorionic placentas the risk of vascular anastomoses could result in disruption that compromises the fetus. These hemodynamic abnormalities are more prevalent after the death of one co-twin; however, it can happen even in surviving infants. This process of hypoxia and ischemia could affect several organs such as the brain (microcephaly, hydrocephalus or hydranencephaly), the gastrointestinal system (intestinal atresia), the kidney, and the skin (aplasia cutis) [112].

Malformations in twins affect the abdominal wall, skull, and chest, as well as the cardiac, musculoskeletal, urogenital and central nervous systems. They are related to embryonic midline fates (neural tube and cardiac defects), hemodynamic instability of the placenta (brain lesions, limb reduction, cardiac defects, renal agenesis, aplasia cutis and intestinal atresia), and anomalies associated with prematurity (patent ductus arteriosus and retinopathy) [113].

The management of discordant anomalies in monochorionic twins is a great challenge when parents decide to keep the pregnancy. The normal fetus is at increased risk of prematurity and its consequences. The major problem occurs after the death of the discordant fetus for con‐ genital anomalies, which increases the risk of death of the normal co-twin around 10-25%. The risk of brain lesions in the surviving infant is approximately 25% [114]. Also, the rate of perinatal death in twins associated with congenital malformations is approximately 15% [115, 116]. Therefore, it is very important to maintain a strict surveillance during the prenatal in order to diminish the risks for the normal co-twin.
