**6. Placental characteristics in monochorionic twins**

Monochorionic placentation is associated with higher perinatal morbidity and mortality as a result of placental morphologic characteristics and vascular problems (Figure 5) [48]. Overall, almost 1% of all monozygotic twin gestations are monoamniotic, which consist of both single amniotic cavity and placenta, sharing two umbilical cord insertions. This may lead to a complication specific to monoamniotic twins: cords entanglement and knotting [49]. For decades it was believed that cord entanglement was responsible for most fetal deaths, but recent studies, including a systematic review, showed no contribution of cord entanglement to prenatal morbidity and mortality [50, 51].

Superficial vascular anastomoses are present in all monoamniotic placentas, with the majority being of arterioarterial and arteriovenous type. Also, a small distance between cords' insertion are observed in most cases, as well as a low incidence of velamentous cord insertion (4%). No significant association among various morphologic or histophatologic characteristics of monochorionic monoamniotic placentas and perinatal mortality were reported. Furthermore, no relation between severe birth weight discordance (≥20%) and unequally shared placenta or velamentous cord insertion were described. Twin-twin transfusion syndrome is a rare condition in monochorionic monoamniotic placentas due to the protector effect of the arterio‐ arterial anastomoses [49, 52].

Likewise, monochorionic diamniotic placentas did not demonstrate a clear relation between placental angioarchitecture, intercord distance and shared placental territories with greater perinatal mortality. Twins with unequally shared placentas and velamentous cord insertion significantly lower mean birth weight. Perinatal mortality was found to be substantially higher in the presence of velamentous cord insertion [48].

Additionally, in cases of TTTS, vascular anastomoses are more likely to be of deep other than superficial type. Most anastomoses are arteriovenous, and vascular communications are fewer in number without compensating superficial arterioarterial flow [53]. Moreover, evidences suggest that unequally shared placentas and velamentous cord insertion are not mandatory for the occurrence of TTTS [54].

**Figure 5.** Monochorionic placentas after injection with coloured dye. The veins are coloured yellow or orange and the arteries are black or blue. The white arrows indicate arteriovenous anastomoses from twin I towards twin II; the black arrows indicate arteriovenous anastomoses from twin II towards twin I. The white arrowheads indicate venovenous anastomoses and the black arrowheads indicate arterioarterial anastomoses. **(A)** Placenta of monochorionic twin without TTTS, delivered at 36+6 weeks of gestation. Similar placental territory for both twins. **(B)** Placenta of mono‐ chorionic twin with selective intrauterine growth restriction. Caesarean section at 32 weeks of gestation after determi‐ nation of lung maturity. The growth-restricted twin I has a velamentous cord insertion and placental territory of 28%. **(C)** Placenta of monochorionic twin with mild TTTS. Caesarean section at 32 weeks of gestation due to TTTS. The exrecipient twin I has a placental territory of 82%. **(D)** Placenta of monochorionic twin with TTTS, conservative manage‐ ment. Caesarean section at 31+5 weeks for signs of anemia in the donor twin. Adapted from Placenta, *Hack et al.* [48], with permission from the publisher.

#### **7. Antenatal care in monochorionic pregancies**

The antenatal chorionicity determination is remarkably precise. Yet, eventual mistakes have a major impact on patient counseling, pregnancy monitoring and perinatal outcome. Some researchers are going for 3-D ultrasound but its contribution for chorionicity determinations

Recognition of zygosity is more difficult to be predicted and can be either performed by ultrasound or noninvasive molecular genetic tests. Only 55-65% of twin pregnancies zygosity can be determined by correlating chorion type with the sex of twins [45, 46]. Invasive ap‐ proaches combined with microsatellite DNA markers could also detect zygosity, but they have the inconvenience of a miscarriage risk of 0.5-1%. Recently, Zheng et al [47] developed a noninvasive method based on maternal plasma target region sequencing through a bioinfor‐

Monochorionic placentation is associated with higher perinatal morbidity and mortality as a result of placental morphologic characteristics and vascular problems (Figure 5) [48]. Overall, almost 1% of all monozygotic twin gestations are monoamniotic, which consist of both single amniotic cavity and placenta, sharing two umbilical cord insertions. This may lead to a complication specific to monoamniotic twins: cords entanglement and knotting [49]. For decades it was believed that cord entanglement was responsible for most fetal deaths, but recent studies, including a systematic review, showed no contribution of cord entanglement

Superficial vascular anastomoses are present in all monoamniotic placentas, with the majority being of arterioarterial and arteriovenous type. Also, a small distance between cords' insertion are observed in most cases, as well as a low incidence of velamentous cord insertion (4%). No significant association among various morphologic or histophatologic characteristics of monochorionic monoamniotic placentas and perinatal mortality were reported. Furthermore, no relation between severe birth weight discordance (≥20%) and unequally shared placenta or velamentous cord insertion were described. Twin-twin transfusion syndrome is a rare condition in monochorionic monoamniotic placentas due to the protector effect of the arterio‐

Likewise, monochorionic diamniotic placentas did not demonstrate a clear relation between placental angioarchitecture, intercord distance and shared placental territories with greater perinatal mortality. Twins with unequally shared placentas and velamentous cord insertion significantly lower mean birth weight. Perinatal mortality was found to be substantially higher

Additionally, in cases of TTTS, vascular anastomoses are more likely to be of deep other than superficial type. Most anastomoses are arteriovenous, and vascular communications are fewer in number without compensating superficial arterioarterial flow [53]. Moreover, evidences suggest that unequally shared placentas and velamentous cord insertion are not mandatory

is still unclear. Further studies should be encouraged [39, 40].

**6. Placental characteristics in monochorionic twins**

matics' model with promising results.

210 Contemporary Gynecologic Practice

to prenatal morbidity and mortality [50, 51].

in the presence of velamentous cord insertion [48].

arterial anastomoses [49, 52].

for the occurrence of TTTS [54].

Multiple pregnancies impose a higher risk of complications for both mother and baby; therefore, adverse outcomes take place more often [55]. Intensive antenatal care should be provided along with a multidisciplinary team. Furthermore, an effective interpersonal communication between healthcare professionals and women is fundamental [56].

The very first step for quality assistance is an early detection of multiple pregnancies along with appropriate amnionicity and chorionicity determination as soon as possible. Whenever the diagnosis of chorionicity is uncertain, the woman should be referred to a specialist or a senior ultrasonographer before 14 weeks. If still indeterminate, even after referral, the pregnancy should be managed as monochorionic until proven otherwise [55-57]. Parents should be thoroughly informed about the implications of a monochorionic pregnancy [58].

Nuchal translucency should be offered as a screening for fetal aneuploidies. The detection accuracy is better when combining maternal age, nuchal fold, crown-rump length, and serum markers [42, 55]. Some professionals do not recommend the routine use of serum markers neither in the first trimester nor during the second trimester [59], while others do recommend for both situations [56].

The prevalence of congenital anomalies is almost twice when comparing monochorionic twins with dichorionic, although in both cases only one fetus is affected in 90% of the time. In case of a suspicious screening exam, a fetal echocardiographic assessment should be considered. The same applies for in vitro fertilization conceived twins and cases of severe TTTS [42]. First trimester surveillance for TTTS is not advised [55, 56, 58]. When applica‐ ble, chorionic villus sampling is preferred over amniocentesis and the transabdominal route is the best choice [59].

Placental evaluation and cervical length assessment are also important. Placenta previa is 40% more common in twins, and so is vasa previa. The placental cord insertion should be deter‐ mined once velamentous cord insertion is associated with greater risk of TTTS, unequal placental sharing and perinatal mortality. Cervical length smaller than 20-25 mm raises the likelihood of preterm delivery in 3-5 times [42].

Serial sonographic monitoring for intrauterine growth restriction (IUGR) or discordance is warranted rather than abdominal palpation, symphysis-fundal height measurement or umbilical artery Doppler [42, 55, 56]. Only an estimated fetal weight discordance greater than 25% is clinically important [55, 56]. Both IUGR and twin discordance are associated with increased risk for fetal and perinatal death [42]. A recent prospective cohort study showed that twin birth weight discordance might be predicted with an abdominal circumference ratio cutoff of 0.93, with a sensitivity and specificity of 61% and 84%, respectively [60].

Additionally, it is also mandatory to monitor for maternal complications, especially for hypertensive disorders that present an increased likelihood of 2 to 3-fold. Concerning gesta‐ tional diabetes, whether its occurrence is increased or not is still controversial. The manage‐ ment of all maternal complications shall not be different from singleton pregnancies [55].

Table 1 shows an overview of ultrasound applications for twin pregnancies [42]. Monthly prenatal consultations are strongly recommended for all monochorionic pregnancies, as well as ultrasound scanning every 4 weeks for uncomplicated dichorionic pregnancy and every 2 weeks for uncomplicated monochorionic twins [42, 59].


From Seminars in Perinatology, *Lynn Simpson* [42], with permission from the publisher.

**Table 1.** Ultrasound in twins

The very first step for quality assistance is an early detection of multiple pregnancies along with appropriate amnionicity and chorionicity determination as soon as possible. Whenever the diagnosis of chorionicity is uncertain, the woman should be referred to a specialist or a senior ultrasonographer before 14 weeks. If still indeterminate, even after referral, the pregnancy should be managed as monochorionic until proven otherwise [55-57]. Parents should be thoroughly informed about the implications of a monochorionic pregnancy [58].

Nuchal translucency should be offered as a screening for fetal aneuploidies. The detection accuracy is better when combining maternal age, nuchal fold, crown-rump length, and serum markers [42, 55]. Some professionals do not recommend the routine use of serum markers neither in the first trimester nor during the second trimester [59], while others do recommend

The prevalence of congenital anomalies is almost twice when comparing monochorionic twins with dichorionic, although in both cases only one fetus is affected in 90% of the time. In case of a suspicious screening exam, a fetal echocardiographic assessment should be considered. The same applies for in vitro fertilization conceived twins and cases of severe TTTS [42]. First trimester surveillance for TTTS is not advised [55, 56, 58]. When applica‐ ble, chorionic villus sampling is preferred over amniocentesis and the transabdominal route

Placental evaluation and cervical length assessment are also important. Placenta previa is 40% more common in twins, and so is vasa previa. The placental cord insertion should be deter‐ mined once velamentous cord insertion is associated with greater risk of TTTS, unequal placental sharing and perinatal mortality. Cervical length smaller than 20-25 mm raises the

Serial sonographic monitoring for intrauterine growth restriction (IUGR) or discordance is warranted rather than abdominal palpation, symphysis-fundal height measurement or umbilical artery Doppler [42, 55, 56]. Only an estimated fetal weight discordance greater than 25% is clinically important [55, 56]. Both IUGR and twin discordance are associated with increased risk for fetal and perinatal death [42]. A recent prospective cohort study showed that twin birth weight discordance might be predicted with an abdominal circumference ratio

Additionally, it is also mandatory to monitor for maternal complications, especially for hypertensive disorders that present an increased likelihood of 2 to 3-fold. Concerning gesta‐ tional diabetes, whether its occurrence is increased or not is still controversial. The manage‐ ment of all maternal complications shall not be different from singleton pregnancies [55].

Table 1 shows an overview of ultrasound applications for twin pregnancies [42]. Monthly prenatal consultations are strongly recommended for all monochorionic pregnancies, as well as ultrasound scanning every 4 weeks for uncomplicated dichorionic pregnancy and every 2

cutoff of 0.93, with a sensitivity and specificity of 61% and 84%, respectively [60].

for both situations [56].

212 Contemporary Gynecologic Practice

is the best choice [59].

likelihood of preterm delivery in 3-5 times [42].

weeks for uncomplicated monochorionic twins [42, 59].
