The 11-14 weeks scan - KH Nicolaides, NJ Sebire, RJM Snijders, AP Souka

Chapter 4

DETERMINATION OF ZYGOSITY AND CHORIONICITY

Zygosity can only be determined by DNA fingerprinting, which requires amniocentesis, chorion villus sampling or cordocentesis.

Determination of chorionicity can be performed by ultrasonography and relies on the assessment of fetal gender, number of placentas, and characteristics of the membrane between the two amniotic sacs. Different-sex twins are dizygotic and therefore dichorionic, but in about two-thirds of twin pregnancies the fetuses are of the same sex and these may be either monozygotic or dizygotic. Similarly, if there are two separate placentas the pregnancy is dichorionic, but in the majority of cases the two placentas are adjacent to each other and there are often difficulties in distinguishing between dichorionic-fused and monochorionic placentas.

In dichorionic twins the inter-twin membrane is composed of a central layer of chorionic tissue sandwiched between two layers of amnion, whereas in monochorionic twins there is no chorionic layer present. The best way to determine chorionicity is by an ultrasound examination at 6-9 weeks of gestation. Dichorionic twins can be easily distinguished by the presence of a thick septum between the chorionic sacs. This septum becomes progressively thinner to form the chorionic component of the inter-twin membrane, but remains thicker and easier to identify at the base of the membrane as a triangular tissue projection, or lambda sign.

Sonographic examination of the base of the inter-twin membrane at 10-13+6 weeks of gestation for the presence or absence of the lambda sign (Figure 2) provides reliable distinction between dichorionic and monochorionic pregnancies. With advancing gestation there is regression of the chorion laeve and the ‘lambda’ sign becomes progressively more difficult to identify. Thus by 20 weeks only 85% of dichorionic pregnancies demonstrate the lambda sign. Therefore, absence of the lambda sign at 20 weeks, and presumably thereafter, does not constitute evidence of monochorionicity and does not exclude the possibility of dichorionicity or dizygosity. Conversely, because none of the pregnancies classified as monochorionic at the 10-13+6 week scan subsequently develop the lambda sign, the identification of this feature at any stage of pregnancy should be considered as evidence of dichorionicity.