Multiple Pregnancy: Twins and Chorionicity

Determining Chorionicity on Early Ultrasound

Chorionicity strongly dictates the ultimate survival rate and specific complication pathways. It is uniquely easiest to confidently determine during the standard first-trimester dating scan (between exactly 11 and 14 weeks gestation).

• Dichorionic Diamniotic (DCDA): These twins physically possess two entirely separate placentas and exactly two distinct amniotic sacs. Nearly all dizygotic (fraternal) twins naturally form as DCDA. It features the lowest complication rate among twin types as the structurally distinct placental circulations are completely independent.
• Monochorionic Diamniotic (MCDA): These twins share a single, vastly larger merged placenta but reside within two distinct, totally separate amniotic fluid sacs. They are unequivocally monozygotic (identical) twins.
• Monochorionic Monoamniotic (MCMA): Extremely treacherous. The fetuses share precisely one placenta and absolutely one single sac. They are at tremendous risk for catastrophic umbilical cord entanglement and sudden fetal demise.

Sonographic T-Sign and Lambda Sign

Evaluating the exact insertion point where the thin dividing inter-twin separation membrane strictly meets the bulky chorionic placenta yields highly definitive radiological signs.

• The Lambda Sign (Twin Peak Sign): A pathognomonic visual hallmark of entirely Dichorionic structural twinning. Because two separate thick chorions meet at the placenta, genuine solid chorionic tissue physically extends upward directly into the base of the deeply dividing membrane, forming a dense, triangular, explicitly highly reflective white 'peak' or Greek letter Lambda (λ).
• The T-Sign: A hallmark strictly defining Monochorionic development. The incredibly thin intervening membrane consists literally only of two utterly fragile layers of amnion without any thick intervening chorion. It structurally meets the shared flat placental surface at an exact, harsh 90-degree right angle, mimicking the sharp letter 'T'.