Multiple pregnancies, teins and their fetal membranes

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

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.

---

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 (sharing up to 50% of genes).

• Always have two separate placenta, two chorions, and two amnions (dichorionic diamniotic: DCDA).
• Placentas might be fused but remain histologically separated.

→ Risk of congenital anomalies is similar to singleton pregnancy.

---

2. Monozygotic (Identical) Twins

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

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

• Splitting at Day 1–3 (during Morula stage) → DCDA

Having…

• Two chorions

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

---

(during Blastocyst stage) → Monochorionic Diamniotic (MCDA)

having…

• Single chorion

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

TTTS

---

(Implanted blastocyst) → MCMA

Having…

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

---

• Splitting after Day 13 → Conjoined twins

• May result in incomplete separation
• Shared organs according to the plane of division → Extremely rare, but a classic embryological example of late twinning failure
  

---

Fetal Membranes in Twins

Twins differ primarily by their numbers of chorion and amnion in membranes

1. Dichorionic Diamniotic (DCDA)

Could be…

• Dizygotic twins (100%)
• Monozygotic twins if splitting in early stage (1–3 days)

Features…

• Two chorions
• Two amnions
• Two placenta(may be fused)
• Thick inter-twin membrane

DCDA twins

↳Composition: 4layers(chorion + amnion + chorion + amnion)

Clinical implications:

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

---

2. Monochorionic Diamniotic (MCDA)

Features…

• One chorion covering both together
• Two amnions
• One placenta(can result in uneven shring of placenta)
• Thin dividing membrane

↳Composition: amnion + amnion (2 layers of amnion only)

MCDA twins

Risks…

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

---

3. Monochorionic Monoamniotic (MCMA)

Features…

• One chorion
• One amnion
• One placenta
• 

  MCMA twins
  NO dividing membrane between

Risks…

• Umbilical cord entanglement
• Increased perinatal mortality

---

4. Conjoined Twins

Features…

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

.

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

Conclusion

Twins 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.

---

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.