Multiple pregnancies, teins and their fetal membranes

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Multiple Pregnancies, Twins and Their Fetal Membranes

Introduction

Multiple pregnancy refers to the development of more than one embryo and fetus within the uterus simultaneously.

The most common form is twin, which can be classified according to their zygocity (monozygotic vs dizygotic) and their fetal membranes.(chorion and amnion)

Understanding the embryological mechanisms behind twin formation is crucial for predicting placentation, fetal risks, and associated obstetric outcomes.

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Types of Twins (Zygosity)

1. Dizygotic (Fraternal) Twins

• Formed from two different oocytes fertilized by two different sperms.
• Represent 70–75% of all twins.
• Genetically similar to normal siblings (share up to 50% of genes).
• Always have two separate placenta, two chorions, and two amnions (dichorionic diamniotic: DCDA).
• Placentas may fuse but remain histologically separated.

➡ Risk of congenital anomalies is similar to singleton pregnancy.

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2. Monozygotic (Identical) Twins

Monozygotic twins arise from one fertilized egg which splits during early development.

Their fetal membranes strictly depend on “when the zygote divides”

Having…

• Two chorions

• Two amnions
• Two placenta (might be fused) ➡ About 25–30% of monozygotic twins ➡ Lowest risk among monozygotic twins

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having…

• Single chorion

• Two amnions
• One shared placenta ➡ Most common form (60–70%) ➡ Higher risk of twin-twin transfusion syndrome (TTTS)

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Having…

• One chorion
• One amnion
• One placenta ➡ Rare (<1%) but very high risk due to cord entanglement

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• Splitting after Day 13 → Conjoined twins

• Incomplete separation
• Shared organs vary according to the plane of division ➡ Extremely rare, but this is a classic embryological example of late twinning failure
  

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Fetal Membranes in Twins

Twins differ primarily by their numbers of chorion and amnion

1. Dichorionic Diamniotic (DCDA)

Could be…

• DZ twins (100%)
• MZ twins if early splitting (1–3 days)

Features…

• Two chorions
• Two amnions
• Two placenta(may be fused)
• Thick intertwin membrane
  • Composition: 4layers

(chorion + amnion + chorion + amnion)

Clinical implications:

• Lowest complication rates
• Essentially “two separate pregnancies” inside of one uterus

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2. Monochorionic Diamniotic (MCDA)

Features…

• One chorion
• Two amnions
• One placenta
• Thin dividing membrane
  • Composition: amnion + amnion (2 layers)

Risks…

• Twin-Twin Transfusion Syndrome (TTTS) due to vascular anastomoses
• Unequal placental sharing (selective IUGR)

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3. Monochorionic Monoamniotic (MCMA)

Features…

• One chorion
• One amnion
• One placenta
• No dividing membrane

Risks…

• Umbilical cord entanglement
• Increased perinatal mortality

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4. Conjoined Twins

Features…

• Single amniotic sac
• Shared organs depending on axis of incomplete separation
• Classified as thoracopagus, omphalopagus, craniopagus, etc.

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Formation of Fetal Membranes (Embryology Overview)

Chorion

Develops from…

• Extraembryonic mesoderm
• Cytotrophoblast
• Syncytiotrophoblast

Functions…

• Formation of villi
• Gas and nutrient exchange
• Determines the type of placentation in twins

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Amnion

Develops from…

• Epiblast cells forming the amniotic membrane

Functions…

• Protective fluid-filled environment
• Prevents adhesion
• Allows fetal movement and lung development

In twins,

amnion number = time of splitting.

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Placental Variations in Twins

• 
  Two placentas (DCDA)
• One placenta with two umbilical cord insertions (MCDA / MCMA)
• Shared vasculature → TTTS
• Unequal placental territory → selective growth restrictions

Histology of placenta is showing…

• Vascular anastomoses
• Differences in villous maturation between the two sides
• Possible fused chorionic plates

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Complications Associated With Twin Pregnancies

• Twin-to-twin transfusion syndrome (TTTS)
• Twin anemia–polycythemia sequence (TAPS)
• Cord entanglement (MCMA)
• Preterm birth
• Growth restriction
• Congenital anomalies (higher in MZ twins)

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Conclusion

Twin and multiple pregnancies exhibit diverse embryological origins, resulting in distinct fetal membrane arrangements and placentation types.

Understanding the timing of embryonic splitting provides a reliable method for predicting chorionicity and amnionicity, which are essential for clinical management and risk assessment.

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Sources

1. Sadler, T. W. Langman’s Medical Embryology. Lippincott Williams & Wilkins.
2. Junqueira & Carneiro. Basic Histology: Text and Atlas. McGraw-Hill.
3. Moore, K. L., Persaud, T. V. N., Torchia, M. The Developing Human: Clinically Oriented Embryology.