Which of the following are true with dizygotic twins?
a. Has two placentas (separate or fused)
b. Communicating vessels are present
c. Has 4 intervening membranes (2 amnions & 2 chorions)
d. Sex may differ

1. A

   A, C, and D only

2. B

   A, B, and C only

3. C

   A, B, C, and D

4. D

   B, C, and D only

Appeared in: MNS 2024

Explanation

• Dizygotic twins result from two separate fertilization events, making them genetically distinct siblings who share a uterus.
• They always have two separate placentas (dichorionic) and two separate amniotic sacs (diamniotic). The placentas may be separate or fused.
• The membrane separating them is thick, composed of four layers: two amnions and two chorions.
• Because they are genetically different, their sex can be the same or different.
• A key feature is the absence of communicating blood vessels between their separate placental circulations, which distinguishes them from monochorionic monozygotic twins.

Why Other Options Were Wrong

• Option B: This option includes statement B, which is false. Communicating vessels are not present in dizygotic twins because they have separate placental circulations.
• Option C: This option includes statement B, which is false. Dizygotic twins lack communicating vessels.
• Option D: This option incorrectly includes statement B (false) and excludes statement A (true). Dizygotic twins always have two placentas and never have communicating vessels.

Related Visual

Clinical Relevance

• Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Characteristics of Dizygotic Twinning as background academic context rather than a clinical decision trigger.
• Determining chorionicity (number of placentas) via early ultrasound is a critical nursing and obstetric priority. It dictates the level of monitoring required during the pregnancy.
• Dichorionic (dizygotic) twin pregnancies are generally lower risk than monochorionic pregnancies, which are susceptible to complications like Twin-to-Twin Transfusion Syndrome (TTTS) due to shared vascular connections.
• Nurses educate expectant parents about the different types of twins and the specific monitoring plan for their pregnancy, including more frequent growth scans and assessments for monochorionic twins.

How to Approach the Question

• First, understand the question is asking for the true characteristics of dizygotic (fraternal) twins.
• Recall the fundamental biology of dizygotic twinning: two eggs fertilized by two sperm.
• Evaluate each statement individually based on this biological origin:
• Statement A (Placenta): Two zygotes mean two implantations, leading to two placentas (dichorionic). True.
• Statement B (Vessels): Separate placentas mean separate blood supplies. No communicating vessels. False.
• Statement C (Membranes): Each fetus has its own chorion and amnion. The separating membrane is therefore thick (4 layers). True.

Concept Tested & Keywords

• Concept Tested: Characteristics of Dizygotic Twinning
• Stem keywords: dizygotic twins, true
• Lead-in keywords: Which of the following are true
• Negative lead-in flag: false

Question ID

Qi0jwuWf-qV1CA2VgJC8_7

Copy ID

Reference Book

E6 Pathology- ROBBINS & COTRAN PATHOLOGIC BASIS OF DISEASE 10TH Ed p. 1029-1031

E6 Anatomy Grays 43e Vol 1 Part 2 p. 1