*3.2.3. Intertwin membrane characteristics*

The intertwin membrane of a dichorionic pregnancy comprises three layers of three membranes: amnion-chorion-amnion, as the monochorionic pregnancy consists only two layers of amnion. Therefore, intertwin membrane in a dichorionic pregnancy is thicker and more echogenic than the intertwin membrane in monochorionic pregnancies. Measuring the thickness of the membrane can help us define chorionicity: a membrane thicker than 2 mm indicates dichorionicity (positive predictive value: 95%), and if the membrane is thinner than 2 mm, the possibility of monochorionic pregnancy is about 90% [4].

The intertwin membrane has to be carefully detected and if it cannot be visualized, a transvaginal ultrasound scan has to be performed, to set the definitive diagnosis of monoamniotic pregnancy [4]. When a single placental mass is visualized and chorionicity is identified as monochorionic, evaluation of the intertwin membrane characteristics is the key to determine amnionicity. The most significant sonographic figure that demonstrates monoamniocity is the demonstration of cord enlargement from the placental or umbilical origin and it is identified easier via color Doppler. Other important findings intimating monoamnionicity are the entanglement of limbs or observation of a limb circumscribing the other, the failure to find the membrane between the two cord insertions in the placenta [4], and the short intercord distance [5].

However, intertwin membrane thickness difference between monochorionic and dichorionic pregnancy decreases during gestation [36]. In addition, the measurement of the thickness of the membrane is not widely accepted since this parameter can be affected by many factors such as the position and the quality of the probe, and as a result, it has poor reproducibility [37]. A rare but significant pitfall may lead to a wrong determination of a monochorionic pregnancy as dichorionic is the intrauterine synechiae in twin pregnancy with a fetus with anencephaly. Intrauterine synechiae can mimic the thick dichorionic membrane [38]. This septum is not the intertwin membrane and does not include the layer of chorion between the layers of amnion.

#### *3.2.4. The chorionic peak sign—the "λ" sign*

*3.2.2. Number of distinct placentas*

**Figure 4.** Nondisjunction of chromosome Y.

72 Multiple Pregnancy - New Challenges

It is logical that the visualization of two separate placental masses confirms dichorionicity as a single placenta identifies monochorionicity [4]. Careful ultrasound evaluation has to be done

As the pattern above, monochorionic twins may form a bipartite placenta. This sonographic finding is visible in 3% of monochorionic twin pregnancies. As a result, two separated placental masses are present with two nearly equal-sized placental lobes, which can be totally separated or connected by chorion laeve. Things can be more complicated when each placental mass has its own umbilical cord connection. Bipartite placenta can be distinguished from the dichorionic placental masses by using color Doppler and identifying vascular anastomoses that are present between the two lobes. Thus, this leads to the conclusion that if an ostensibly dichorionic pregnancy is complicated with TTTS, the diagnosis of a monochorionic

in order to define the presence of a single placenta or two placentas in abutment.

pregnancy with bipartite placenta has to be considered [32–35].

The chorionic peak sign or the "λ" sign supports strongly dichorionicity, with an accuracy of 99% [5]. It shows a projecting zone of tissue which is as echogenic as the placenta; it has a triangular shape in cross-section; and it is wider at the chorionic surface of the placenta, extending into, and tapering to a point within, the intertwin membrane [39, 40]. The absence of the "λ" sign or the presence of "T" sign indicates monochorionicity. The "T" sign represents the two opposing amnions "standing" at the base of the intertwin membrane [10].

The chorionic peak sign is ideally evaluated during the late first trimester or the very early second trimester, as in second trimester, it is more difficult to be visualized and it might be disappeared at 16–20 weeks of gestation, leading to a false negative "λ" sign. As a result, the impossible depiction of the "λ" sign in late second trimester cannot exclude dichorionicity [41, 42]. Nonetheless, a false positive "λ" sign might also exist. This can be due to umbilical cord insertion into the intertwin membrane or because of the visualization of a hematoma presented along the insertion of the membrane. Another interesting reason that may lead to a false positive "λ" sign is the presence of an echogenic retrograded yolk sac of the placental junction of the intertwin membrane in a monochorionic-diamniotic twin gestation. The sonographic finding that succors determinate the true "λ" sign is that the true "λ" has been seen along with the whole insertion area, in contrast to the false "λ" sign, which appears in only a small region of the intertwin membrane [43, 44]. Finally, in very rare instances, the placentation may be both monochorionic and dichorionic, and each chorionicity is presented in different regions of the intertwin membrane. Therefore, the same intertwin membrane has parts with two layers of amnions and parts with three layers: amnion-chorion-amnion [45–47]. This situation shows the importance of scanning the whole insertion of the intertwin membrane in early ultrasound assessment of multiple pregnancy.

**5. Conclusion**

tertiary center could be really valuable.

**Acknowledgements**

**Author details**

Panagiotis Antsaklis1

George Daskalakis1

**References**

There is no doubt that multiple pregnancies are now more frequent than a few years before, due to the spreading of artificial reproductive technologies. Determination of gestational age, chorionicity, and amnionicity has to be done as soon as possible and ideally in the first trimester of the pregnancy, as the accuracy of the determining sonographic figures is extremely close to 100%, in contrast to the definition in the second trimester whose accuracy is slightly decreased. Last but not least, timely determination of both chorionicity and amnionicity can optimize the outcome of the pregnancy, as the correct and early intervention or a refer to a

We would like to thank Kyriaki Savva, PhD student of Cyprus Institute of Neurology and

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[2] Kurtz GR, Keating WJ, Loftus JB. Twin pregnancy and delivery: Analysis of 500 twin

[3] Glanc P, Nyberg DA, Khati NJ, Deshmukh SP, Dudiak KM, Henrichsen TL, Poder L, Shipp TD, Simpson L, Weber TM, Zelop CM. ACR Appropriateness Criteria® Multiple

[4] Morin L, Lim K. Ultrasound in twin pregnancies. Journal of Obstetrics and Gynaecology

Gestations. Journal of American College of Radiology. 2017;**14**(11):S476-S489

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\*, Maria Papamichail<sup>2</sup>

1 Alexandra Maternity Hospital, University of Athens, Athens, Greece

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3 Obstetrics and Gynecology, Alexandra Maternity Hospital, Athens, Greece

\*Address all correspondence to: panosant@gmail.com

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2 University of Athens, Athens, Greece

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In some cases and despite the best possible ultrasound assessment, chorionicity is impossible to be defined. In these situations, the pregnancy has to be considered as monochorionic. Therefore, surveillance has to be as close as in monochorionic pregnancies [45], and this is discussed below.
