Ultrasonic determination of chorionicity

Chorionicity can be determined on ultrasound with 100% accuracy in the first trimester, by counting the constituent layers of the dividing membranes. Thick chorion is obvious in a DC intertwin septum, while the tissue paper thin amnion is normally resolved separately from the chorion in the first trimester (Fig. 20.3). A simple method involves inspecting the placental base of the membrane which has a thick 'lambda' shape if DC, or a thin 'T' shape if MC. Finally, a single extra-embryonic coelom with two yolk sacs confirms MC diamniotic placentation,

Dividing Membrane Ultrasound

Fig. 20.3 First trimester chorionicity determination. (a) A thick intertwin septum, with each twin's amnion (arrows) identified separately from the chorion, indicating dichorionicity. (b) A thin intertwin septum, which diverges (arrows) into a single extra-embryonic coelom, indicating the absence of intervening chorion and thus monochorionicity (reproduced with permission from [1]).

Fig. 20.3 First trimester chorionicity determination. (a) A thick intertwin septum, with each twin's amnion (arrows) identified separately from the chorion, indicating dichorionicity. (b) A thin intertwin septum, which diverges (arrows) into a single extra-embryonic coelom, indicating the absence of intervening chorion and thus monochorionicity (reproduced with permission from [1]).

whereas a single coelom with a single yolk sac and no dividing septum indicates monoamnionicity. In the mid-trimester, after the thinned chorion leave has fused with the amnion, chorionicity determination is only 80-90% accurate. Qualitative interpretation as thick (DC) or thin (MC) appears as accurate as septal measurement [12]. Discordant external genitalia indicate dizygosity and thus dichorionicity and separate placentae dichorionicity, as does demonstration of a tongue of placental tissue within the base of the septum known as the 'twin peak' sign. In contrast a thin septum in concordant-sex twins with a single placental mass suggests MC. Demonstration by colour Doppler of a functional artery-to-artery anastomosis is more challenging but provides definitive proof of monochorionicity.

Chorionicity should be determined on ultrasound in all multiple pregnancies [13]. This is because chorionicity is relevant to: (1) counselling parents about perinatal risks; (2) counselling parents about their risk of genetic and structural abnormality; (3) invasive testing and management of discordant abnormality; (4) feasibility of multifetal pregnancy reduction; (5) risk of sequelae in the presence of fetal compromise; (6) early detection and management of twin-twin transfusion syndrome. It should be done at the first ultrasound, as it is most accurate in the first trimester; fortunately all ART pregnancies have an early scan, as do the increasing number undergoing nuchal screening.

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  • GRINGAMOR
    When can chorionicity be determined by ultrasound?
    4 years ago

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