Chorionicity and Amnionicity in Twins





KEY FACTS


Terminology





  • 70% dizygotic : 2 ova fertilized by different sperm; therefore, all are dichorionic



  • 30% monozygotic : Single ovum fertilized by single sperm, which then divides to produce twins




    • 30% dichorionic, 60-65% monochorionic diamniotic



    • 5-10% monoamniotic, < 1% conjoined




Imaging





  • Dichorionic 1st trimester: Thick echogenic chorion completely surrounds each embryo




    • 2 amniotic sacs, 2 yolk sacs (YS)




  • Dichorionic 2nd/3rd trimester: Twin peak sign with wedge of chorionic tissue extending from placenta into base of intertwin membrane




    • Thick intertwin membrane, 2 separate placentas




  • Monochorionic 1st trimester: Single thick chorion surrounds both embryos




    • Diamniotic: 2 YS, 2 amnions, 1 around each embryo



    • Monoamniotic: 1 YS, 1 amnion surrounds both embryos




  • Monochorionic 2nd/3rd trimester must be same sex with single placental mass




    • Diamniotic: Thin membrane, T sign of membrane perpendicular to placenta



    • Monoamniotic: No membrane, cord entanglement




Scanning Tips





  • Use transvaginal US in 1st trimester to document chorionicity and amnionicity as chorionicity determines prognosis



  • Look for placenta/vasa previa, marginal/velamentous cord



  • Monitor growth at least monthly



  • Check fluid volume every 2 weeks in monochorionic twins to monitor for twin-twin transfusion




    • If diamniotic: Maximum vertical pockets on either side of membrane



    • Must rely on bladders/Doppler in monoamniotic




  • Careful search for anomalies; increased risk in all twins







Graphic of dichorionic twins shows a thick intertwin membrane composed of 2 thin layers of amnion and 2 thick layers of chorion . The placentas are separate.








Transvaginal ultrasound in the 1st trimester shows 2 thick, echogenic, chorionic sacs , each surrounding an embryo . The twin peak sign and thick membrane will develop as a result of apposition of the adjacent chorions . This case illustrates the ease of determining chorionicity with vaginal sonography.








Transabdominal ultrasound at a later gestational age shows 2 embryos with the delicate amniotic membranes barely visible inside the echogenic chorionic sacs. The twin peak sign is now visible where the broad-based triangle of chorion extends into the thick membrane. This is also sometimes called the lambda sign.








Transabdominal ultrasound shows different fetal sex. These twins are therefore dizygotic and, by definition, must be dichorionic.








Graphic of monochorionic diamniotic twins shows a thin membrane formed by the apposition of the 2 thin layers of amnion . There is a single placenta and a single chorionic sac .

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Nov 10, 2024 | Posted by in ULTRASONOGRAPHY | Comments Off on Chorionicity and Amnionicity in Twins

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