Determination of Chorionicity
The distinction between high-risk monochorionic and lower-risk dichorionic twin pregnancies can be accurately achieved in the first-trimester scan, because amnion and chorion are still separated from one another. Chorionicity is therefore simply a matter of counting the layers that separate the twins. In monochorionic diamniotic twin pregnancies, the fetuses are separated by only two thin layers of amniotic membrane. In contrast, dichorionic twins are separated by three layers: two thin amniotic membranes with a thick layer of chorion in between . The lambda sign reflects the apposition of the two placental disks and is also indicative of dichorionicity . Nevertheless, the concept of the lambda sign may be confusing. In first-trimester monochorionic diamniotic twins, the two amniotic sacs and the uterine wall form a lambda rather than a T sign, although the lambda here is an ‘empty’ rather than a ‘full’ lambda.
Later in gestation, because of the close apposition of amnion and chorion, it becomes much more difficult to accurately determine if same-sex twins are mono- or dichorionic. Although the lambda sign usually indicates dichorionicity at later gestational ages, some caution is required because folding of the chorionic plate can create a ‘false’ lambda sign in a monochorionic pregnancy. Nevertheless, as chorionicity is most accurately determined in the first trimester and because nearly all pregnant women have nowadays access to a first-trimester scan, the importance of first-trimester chorionicity determination cannot be stressed enough. It is strongly recommended to store the images of the intertwin septum as proof chorionicity, should there be any doubt later on.
Determination of Chorionicity The distinction between high-risk monochorionic and lower-risk dichorionic twin pregnancies can be accurately achieved in the first-trimester scan, because amnion and chorion are still separated from one another. Chorionicity is therefore simply a matter of counting the layers that separate the twins. In monochorionic diamniotic twin pregnancies, the fetuses are separated by only two thin layers of amniotic membrane. In contrast, dichorionic twins are separated by three layers: two thin amniotic membranes with a thick layer of chorion in between . The lambda sign reflects the apposition of the two placental disks and is also indicative of dichorionicity . Nevertheless, the concept of the lambda sign may be confusing. In first-trimester monochorionic diamniotic twins, the two amniotic sacs and the uterine wall form a lambda rather than a T sign, although the lambda here is an ‘empty’ rather than a ‘full’ lambda. Later in gestation, because of the close apposition of amnion and chorion, it becomes much more difficult to accurately determine if same-sex twins are mono- or dichorionic. Although the lambda sign usually indicates dichorionicity at later gestational ages, some caution is required because folding of the chorionic plate can create a ‘false’ lambda sign in a monochorionic pregnancy. Nevertheless, as chorionicity is most accurately determined in the first trimester and because nearly all pregnant women have nowadays access to a first-trimester scan, the importance of first-trimester chorionicity determination cannot be stressed enough. It is strongly recommended to store the images of the intertwin septum as proof chorionicity, should there be any doubt later on.
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