**5. Conclusion**

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

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.

Surveillance in multiple pregnancies has a significant importance, as it plays the major role in the detection of complications that are associated with a high-risk pregnancy, and it is well known that multiple pregnancy is a classic example of a high-risk pregnancy. However, the appropriate frequency of the ultrasound assessment in both dichorionic and monochorionic pregnancies, which provides the best balance between cost and effectiveness, is not be estab-

Finberg et al. [46] suggested repeat scans every 4–6 weeks for noncomplicated dichorionic pregnancies. However, in current daily medical routine, surveillance is closer: follow-up ultrasound assessments are performed every 3–4 weeks [4, 47]. But, if a complication is suspected, and more specifically when CRL, estimated fetal weight or amniotic fluid volume are different between

It is a well-established fact that surveillance in monochorionic pregnancies has to be closer in relation to a dichorionic pregnancy. Finberg et al. [46] recommended ultrasound monitoring for noncomplicated monochorionic twins every 3–4 weeks. As the pattern mentioned previously, nowadays, routine scans are performed more frequently: they are performed every 2–3 weeks, starting from the gestational age of 16 weeks. Finally, in some cases, surveillance

The parameters that are necessary to be evaluated in these follow-up scans are estimated fetal weight and fetal biometry, amniotic fluid volume, and Doppler assessment of the umbilical

the two fetuses, routine scans have to be repeated every 2 weeks, or within a week [48].

is even closer: a follow-up scan can be repeated every 2 weeks.

early ultrasound assessment of multiple pregnancy.

**4. Surveillance**

74 Multiple Pregnancy - New Challenges

lished and worldwide accepted [3].

**4.1. Dichorionic pregnancies**

**4.2. Monochorionic pregnancies**

artery [49].

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 tertiary center could be really valuable.
