In twin-twin transfusion syndrome, which twin often presents with oligohydramnios on ultrasound?

• A. Donor twin
• B. Recipient twin
• C. Both twins equally
• D. Neither twin

Answer: (A)

In twin-twin transfusion syndrome, one twin (the donor) loses blood to the other twin (the recipient) through placental vascular connections. This imbalance leaves the donor in a hypovolemic state, reducing renal perfusion and urine production, which leads to a decrease in amniotic fluid—oligohydramnios. The recipient, on the other hand, becomes hypervolemic and produces more urine, resulting in polyhydramnios. On ultrasound you typically see discordant amniotic fluid volumes, with the donor showing oligohydramnios (often a small or absent bladder) and the recipient showing polyhydramnios. Thus, oligohydramnios is most commonly seen in the donor twin.