Twin placentas

Twin placentas often come to the pathologist... even if they are normal. In these specimens, usually, the chorion is the key; the pathologist may be able to sort-out whether the twins are monozygotic (identical) or dizygotic (fraternal).

Monozygotic versus dizygotic twins

Monoamniotic-monochorionic gestation

• Abbreviated MoMo.

General

• Split at approximately 7th day.
• Always monozygotic twins.

Gross

• No membrane between the fetuses - apparently clinically.

Diamniotic-monochorionic gestation

• Abbreviated DiMo.

General

• The dogma is that the twins are always monozygotic.
  • There is a case report of an exception.
• Highest risk of TTTS (twin-to-twin transfusion syndrome).

Gross

• Thin membrane at T-zone.

Microscopic

Features:

• One chorion (no decidua) in the T-zone.

Images

• Diamniotic-monochorionic gestation (pathologypics.com).
• Di-Mo placenta (utah.edu).

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TWIN PLACENTA, UMBILICAL CORDS AND FETAL MEMBRANES, BIRTH:
- FETAL MEMBRANES AT T-ZONE:
-- CHORION ABSENT (DIAMNIOTIC-MONOCHORIONIC GESTATION).
- TWIN A:
-- THREE VESSEL UMBILICAL CORD WITHIN NORMAL LIMITS.
-- FETAL MEMBRANES WITHIN NORMAL LIMITS.
-- PLACENTAL DISC WITH THIRD TRIMESTER VILLI WITHIN NORMAL LIMITS.
- TWIN B:
-- THREE VESSEL UMBILICAL CORD WITHIN NORMAL LIMITS.
-- FETAL MEMBRANES WITHIN NORMAL LIMITS.
-- PLACENTAL DISC WITH THIRD TRIMESTER VILLI WITHIN NORMAL LIMITS.

Diamniotic-dichorionic gestation

• Abbreviated DiDi.

General

• Most dizygotic (70%), may be monozygotic (30%).
• If monozygotic -- split before 3 days.

Gross

• Thick membrane at T-zone.

Microscopic

Features:

• Decidua interposed between two chorions.

Images

• Di-Di placenta (utah.edu).

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Normal

TWIN PLACENTA, UMBILICAL CORDS AND FETAL MEMBRANES, BIRTH:
- FETAL MEMBRANES AT T-ZONE:
-- CHORION PRESENT (DIAMNIOTIC-DICHORIONIC GESTATION).
- TWIN A:
-- THREE VESSEL UMBILICAL CORD WITHIN NORMAL LIMITS.
-- FETAL MEMBRANES WITHIN NORMAL LIMITS.
-- PLACENTAL DISC WITH THIRD TRIMESTER VILLI WITHIN NORMAL LIMITS.
- TWIN B:
-- THREE VESSEL UMBILICAL CORD WITHIN NORMAL LIMITS.
-- FETAL MEMBRANES WITHIN NORMAL LIMITS.
-- PLACENTAL DISC WITH THIRD TRIMESTER VILLI WITHIN NORMAL LIMITS.

Early delivery

 TWIN PLACENTA, UMBILICAL CORDS AND FETAL MEMBRANES, CESAREAN SECTION:
- FETAL MEMBRANES AT T-ZONE:
-- CHORION PRESENT (DIAMNIOTIC-DICHORIONIC GESTATION).
- TWIN A:
-- THREE VESSEL UMBILICAL CORD WITHIN NORMAL LIMITS.
-- FETAL MEMBRANES WITHIN NORMAL LIMITS.
-- PLACENTAL DISC WITH FOCALLY MILDLY LARGE THIRD TRIMESTER VILLI AND
   ZONAL CONGESTION.
- TWIN B:
-- THREE VESSEL UMBILICAL CORD WITHIN NORMAL LIMITS.
-- FETAL MEMBRANES WITHIN NORMAL LIMITS.
-- PLACENTAL DISC WITH FOCALLY MILDLY LARGE THIRD TRIMESTER VILLI.

Pathology seen only in twin pregnancies

Twin-to-twin transfusion syndrome

General

• Abbreviated as TTTS.

Definition:

• Monozygotic twins that share a placental disc, have vessels which cross-over between the twins that lead to a blood imbalance between the two twins.
  • Only seen in monozygotic twins.
  • Vascular connection may be vein-to-vein, artery-to-vein, artery-to-artery (uncommon).

Prevalence:

• Seen in ~15% of monozygotic twins.

Clinical:

• Donor:
  • Twin: hypovolemic, oliguric, oligohydramnic, +/- anemia, +/-hypoglycemia, +/- small pale organs.
  • Placental disc: large, pale.
• Recipient:
  • Twin: hypervolemia, polyuria, polyhydramnios, +/- hydrops fetalis, +/- CHF, hemolytic janundice, +/- large congested organs.
  • Placental disc: small, firm, congested.

Gross

• Large vessels that connect the two umbilical cords.

Image:

• 

  Painting thought to depict TTTS. (WC)

Microscopic

Features:

• Artery-to-vein anatomosis - where artery and vein are associated with different umbilical cords.
• Donor twin side of placenta:
  • Edematous villi.
  • Increased nucleated RBCs.
• Recipient twin side of placenta:
  • Congested.

See also

• Placenta.