A 31-year-old woman presents for a dating ultrasound at 12 weeks 3 days with a spontaneously conceived dichorionic twin pregnancy. On transabdominal ultrasound, two separate placental masses are identified on opposite uterine walls. At the point where the inter-twin membrane meets the anterior placenta, a thick triangular tongue of placental tissue is seen projecting into the base of the membrane (marked **A** in the diagram). Which of the following best describes the significance of the structure marked **A** in confirming chorionicity?
A. The lambda or twin-peak sign is pathognomonic for dichorionic diamniotic pregnancy and indicates the highest diagnostic accuracy when assessed in the late first trimester
B. The twin-peak sign represents fusion of two separate amniotic membranes and is equally common in monochorionic and dichorionic pregnancies
C. The presence of a triangular echogenic projection at the membrane-placenta junction indicates monochorionic diamniotic pregnancy with increased risk of twin-to-twin transfusion syndrome
D. The structure represents a placental septum that develops in response to abnormal placentation and is associated with increased perinatal mortality in twin pregnancies
Explanation
Why the lambda or twin-peak sign is pathognomonic for dichorionic diamniotic pregnancy is correct
The lambda or twin-peak sign (marked A) is a thick triangular tongue of placental tissue that projects into the base of the inter-twin membrane at the point where it meets the placenta. According to ISUOG Practice Guidelines, this sign is pathognomonic for dichorionic diamniotic pregnancy and provides the highest diagnostic accuracy when assessed in the late first trimester (11–14 weeks). The presence of this sign, combined with two separate placental masses and a thick inter-twin membrane (>2 mm with multiple visible layers), definitively establishes dichorionic diamniotic chorionicity. This is the lowest-risk twin type, with minimal risk of complications such as twin-to-twin transfusion syndrome, selective intrauterine growth restriction, or monoamniotic cord entanglement.
Why each distractor is wrong
Monochorionic diamniotic pregnancy with TTTS risk: The lambda sign is specific to dichorionic pregnancies. Monochorionic diamniotic twins have a thin inter-twin membrane without the characteristic triangular echogenic projection and carry a 10–15% risk of twin-to-twin transfusion syndrome. The lambda sign's presence explicitly excludes this diagnosis.
Fusion of amniotic membranes equally common in both types: The twin-peak sign is not a simple fusion of membranes and is NOT equally common in monochorionic and dichorionic pregnancies. It is a dichorionic-specific marker formed by the projection of placental tissue (chorionic villi) into the base of the thick dichorionic diamniotic membrane. Its presence is diagnostic of dichorionic pregnancy.
Placental septum associated with increased perinatal mortality: While a septum does separate the two amniotic cavities in dichorionic pregnancy, the lambda sign is not a pathological finding and is not associated with increased perinatal mortality. Dichorionic diamniotic pregnancy has the best prognosis among all twin types. The structure is a normal anatomical marker of dichorionic pregnancy, not a sign of abnormal placentation.
High-YieldNEET PG
The lambda or twin-peak sign at the membrane–placenta junction is pathognomonic for dichorionic diamniotic pregnancy and should be assessed in the late first trimester (11–14 weeks) for highest diagnostic accuracy and optimal pregnancy counselling.
ISUOG Practice Guidelines: role of ultrasound in twin pregnancy
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