What is it?

UPS is a complication that occurs in monochorionic twin pregnancies (twins sharing a single placenta) in which the placenta is not evenly shared by genetically “identical” twins.

How common is it?

Although the exact rate of occurrence is unclear, UPS affects a minority of monochorionic twin pregnancies.

How is it managed?

Beginning at about 16 to 18 weeks of pregnancy, monochorionic twin pregnancies are continuously observed to assess fetal growth and amniotic fluid volume for each twin. When one twin is severely growth-restricted or there is a substantial difference in size between supposedly “identical” twins, a UPS diagnosis must be considered.
UPS places both twins at risk for premature delivery and poor outcomes. In severe cases where testing indicates that a growth-restricted twin appears to be extremely compromised very early in the pregnancy, some patients may consider termination of that twin with the use of Radiofrequency Ablation (RFA). This may improve the chances of a favorable outcome for the normally-grown co-twin.


Outcomes of pregnancies complicated by UPS vary widely, and are influenced by factors including: gestational age at diagnosis, gestational age at delivery, severity of ultrasound findings, whether both twins survive to delivery, and therapies that may be performed before birth. These pregnancies are at high risk for premature delivery, which is a major factor contributing to poor outcomes after delivery.