An ultrasound of a twin pregnancy reveals a single placenta and a thin dividing membrane between the two fetuses. This finding is characteristic of which type of placentation?

1. A

   Dichorionic-Diamniotic

2. B

   Monochorionic-Diamniotic

3. C

   Monochorionic-Monoamniotic

4. D

   Conjoined twins

Appeared in: NORCET 9 Prelims-2025

Explanation

• The term 'monochorionic' refers to the presence of a single, shared placenta, which is stated in the question.
• The term 'diamniotic' refers to the presence of two separate amniotic sacs, indicated by the 'thin dividing membrane' seen on ultrasound.
• Combining these two findings correctly identifies the placentation as monochorionic-diamniotic.
• The provided image clearly illustrates this configuration: two fetuses, each in its own amniotic sac, sharing a single placenta.

Why Other Options Were Wrong

• Option A: Dichorionic-Diamniotic (Di-Di) pregnancies have two separate placentas (or a single fused one) and two amniotic sacs. The dividing membrane is typically thick, and a 'lambda' or 'twin peak' sign is often visible on ultrasound, which contradicts the 'single placenta' and 'thin membrane' described.
• Option C: Monochorionic-Monoamniotic (Mo-Mo) pregnancies have a single placenta and a single amniotic sac that both twins share. A key feature is the absence of any dividing membrane, which contradicts the findings.
• Option D: Conjoined twins are a rare complication of monochorionic-monoamniotic pregnancies where the fetuses are physically fused. It is a type of fetal development anomaly, not a type of placentation itself. The question describes placentation, not fetal anatomy.

Related Visual

• Visual 1: Diagram - A comparative chart showing the three main types of twin placentation (Di-Di, Mo-Di, Mo-Mo) with clear labels for the chorion, amnion, and placenta in each type.
• Visual 2: Ultrasound Images - Side-by-side ultrasound images showing the characteristic 'lambda sign' of a dichorionic pregnancy versus the 'T-sign' of a monochorionic-diamniotic pregnancy.

Clinical Relevance

• Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Identification of twin pregnancy type based on ultrasound findings of placentation as background academic context rather than a clinical decision trigger.
• Determining chorionicity is the single most important step in managing a twin pregnancy because monochorionic twins are at a significantly higher risk for complications.
• Monochorionic twins share a single placenta, leading to vascular anastomoses (shared blood vessels). This creates a risk for Twin-to-Twin Transfusion Syndrome (TTTS), a serious condition where blood flows unequally between the twins.
• Due to the increased risks, monochorionic-diamniotic pregnancies require more frequent ultrasound monitoring (typically every 2 weeks) to screen for TTTS and other complications, compared to dichorionic pregnancies.

How to Approach the Question

• First, break down the terminology in the question. Identify the key findings: 'single placenta' and 'thin dividing membrane'.
• Translate these findings into medical terms. 'Single placenta' corresponds to 'monochorionic' (mono = one, chorion = placenta).
• Translate the second finding. A 'dividing membrane' implies two sacs. This corresponds to 'diamniotic' (di = two, amnion = sac).
• Combine the terms to form the diagnosis: Monochorionic-Diamniotic.
• Evaluate the given options. Select the option that matches your derived diagnosis.
• Use the provided image to confirm your understanding. The image shows one placenta and two sacs, matching the question's description.

Concept Tested & Keywords

• Concept Tested: Identification of twin pregnancy type based on ultrasound findings of placentation.
• Stem keywords: twin pregnancy, ultrasound, single placenta, thin dividing membrane, placentation
• Lead-in keywords: characteristic of
• Clinical cues: The combination of 'single placenta' and 'thin dividing membrane' are the key diagnostic clues.

Question ID

Q0LyXwW_nd6yY9xyuN01VM

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