Multifetal Gestation

Question 1

Different Types of Monozygotic Twin Pregnancy

Answer 1

• Dichorionic-diamniotic: division within 3 days (72 hours) of fertilization
• Monochorionic-diamniotic:
  division after 4-8 days
  (most common)
• Monochorionic-monoamniotic: division after 9-12 days
• Monochorionic-monoamniotic (conjoined twins): division after 13 days

Question 2

Conjoined Twins Classification

Answer 2

• Thoracopagus: anterior union of upper trunk (most common)
• Pygopagus: joining of posterior rumps
• Craniopagus: joining of heads
• Ischiopagus: anterior union of lower 1/2 of body

Question 3

Twin-Twin Transfusion Syndrome

Answer 3

• TTTS occurs in 10-15% of monochorionic twins
• Results from imbalance of vascular anastomoses of placental vessels between fetuses leading to “loss” of volume/blood from 1 twin (donor) to other (recipient)
• Donor twin characteristics:
  often smaller with lower amniotic fluid volume (oligohydramnios); may develop heart failure due to anemia & hypovolemia
• Recipient twin characteristics:
  often larger with higher amniotic fluid volume (polyhydramnios); may develop heart failure due to polycythemia & volume overload

Question 4

Acardiac twin (twin reversed arterial perfusion, TRAP)

Answer 4

• <1% of monochorionic twin pregnancies
• Results from arterial-to-arterial placental anastomosis
• Arterial blood from “pump” twin flows directly into placental artery of “acardiac twin” (in reverse direction)
• Leads to severe malformation (i.e., often no development of structures cephalad of abdomen)
• May lead to high output cardiac failure in pump twin