**5. Diagnosis of chorionicity**

Chorionicity, different from zygosity, refers to the type of placentation and it directly impacts obstetric management (Figure 3) [37]. Distinguishing the placental chorionicity plays a critical role in clinical practice since perinatal mortality rates are 2-5 times higher in cases of monochorionicity, which is present in 20% of all twin pregnancies [37, 38]. Monochorionic placentas may present vascular communications that can induce several syndromes. These vascular anastomoses also explain the existence of chimerism and mosaicism upon monozygotic twins [23].

Correct antenatal assignment of chorionicity is very important not only for risk stratification and prenatal monitoring, but also for genetic counseling, invasive procedures, diagnosis of twin-twin transfusion syndrome (TTTS) and growth abnormalities, as well as for the manage‐ ment of conditions affecting only one twin [39, 40]. Thus, the ascertainment of chorionicity has enabled the prevention of undesired repercussions.

Currently, early sonographic study is the gold standard for the antenatal twin chorionicity prediction. When assessed before 14 weeks' gestation it is extremely precise, with reported accuracy rates ranging from 77 to 100% [40, 41]. Such large variation can be mainly explained by the use of different ultrasound markers and by the time of scanning. Combining firsttrimester sonographic parameters makes it possible to reach accuracy close to 100% [41, 42].

The identification of two clearly separate placentas or gestational sacs during the earliest firsttrimester ultrasound scanning indicates dichorionic twinning, with more than 97% sensibility and 100% specificity. In cases of single or even fused placenta, the chorionicity can be assessed either by the presence of lambda sign or T-sign (Figure 4). Measurement of the inter-twin membrane thickness and counting of the layers of the inter-twin membrane are less useful indicators [37, 42].

In 1981, Bessis and Papiernik [43] first described the lambda sign as a reference for the triangular projection of placental tissue observed at the base of the inter-twin membrane in cases of dichorionic placentation. It has been mutually used with the twin peak sign, described later in 1992 by Finberg [44]. The lambda-sign is better perceived in the late first and early second trimester ultrasound scanning, and may disappear by week 20 in 7% of dichorionic pregnancies with fused placenta [41, 42]. The absence of twin peak sign neither excludes dichorionic pregnancy nor implies monochorionicity [37].

than superfecundation and only few human cases have been described [35]. Confirmation requires ultrasound scanning during the first trimester, but neurosonography with detailed ophthalmic examination may support the diagnosis. Superfetation has innumerous antenatal implications although it is very difficult to retrospectively confirm the diagnosis postnatally [36]. Considering the absence of substantial evidence, we believe the superfetation mechanism

Chorionicity, different from zygosity, refers to the type of placentation and it directly impacts obstetric management (Figure 3) [37]. Distinguishing the placental chorionicity plays a critical role in clinical practice since perinatal mortality rates are 2-5 times higher in cases of monochorionicity, which is present in 20% of all twin pregnancies [37, 38]. Monochorionic placentas may present vascular communications that can induce several syndromes. These vascular anastomoses also explain the existence of chimerism and

Correct antenatal assignment of chorionicity is very important not only for risk stratification and prenatal monitoring, but also for genetic counseling, invasive procedures, diagnosis of twin-twin transfusion syndrome (TTTS) and growth abnormalities, as well as for the manage‐ ment of conditions affecting only one twin [39, 40]. Thus, the ascertainment of chorionicity has

Currently, early sonographic study is the gold standard for the antenatal twin chorionicity prediction. When assessed before 14 weeks' gestation it is extremely precise, with reported accuracy rates ranging from 77 to 100% [40, 41]. Such large variation can be mainly explained by the use of different ultrasound markers and by the time of scanning. Combining firsttrimester sonographic parameters makes it possible to reach accuracy close to 100% [41, 42].

The identification of two clearly separate placentas or gestational sacs during the earliest firsttrimester ultrasound scanning indicates dichorionic twinning, with more than 97% sensibility and 100% specificity. In cases of single or even fused placenta, the chorionicity can be assessed either by the presence of lambda sign or T-sign (Figure 4). Measurement of the inter-twin membrane thickness and counting of the layers of the inter-twin membrane are less useful

In 1981, Bessis and Papiernik [43] first described the lambda sign as a reference for the triangular projection of placental tissue observed at the base of the inter-twin membrane in cases of dichorionic placentation. It has been mutually used with the twin peak sign, described later in 1992 by Finberg [44]. The lambda-sign is better perceived in the late first and early second trimester ultrasound scanning, and may disappear by week 20 in 7% of dichorionic pregnancies with fused placenta [41, 42]. The absence of twin peak sign neither excludes

could only be possible in theory.

208 Contemporary Gynecologic Practice

**5. Diagnosis of chorionicity**

mosaicism upon monozygotic twins [23].

indicators [37, 42].

enabled the prevention of undesired repercussions.

dichorionic pregnancy nor implies monochorionicity [37].

**Figure 3.** Different patterns of placentation for twins. Adapted from Prenatal Diagnosis, *Shetty&Smith* [37], with per‐ mission from the publisher.

The T-sign has been traditionally used to describe the point where the two opposing amnions at the base of the separating membrane approach the placenta at almost a 900 angle, charac‐ terizing a monochorionic placentation [37, 42]. In 2002, Carroll et al [38] performed the very first robust study evaluating sonographic signs between 10-14 weeks of gestation. In their series of 150 cases, the prenatal chorionicity diagnosis was confirmed postnatally by placental histology. They identified a sensitivity and specificity of the T-sign in predicting monochor‐ ionicity of 100% and 98.2%, respectively. The combination of the lambda sign or two separate placentas showed a sensitivity of 97.4% and specificity of 100% to predict dichorionicity. Innumerous studies were subsequently carried out and similar sensitivity and specificity percentages were reported [41].

**Figure 4.** First trimester ultrasound image of a fused dichorionic placenta with lambda sign (A) and first trimester ul‐ trasound image of a monochorionic placenta with T sign (B). Adapted from BJOG, *Carrolletal.* [38], with permission from the publisher.

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

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‐ matics' model with promising results.
