What defines growth discordance in twins?

Growth discordance in twins refers to a significant difference in size between the two fetuses. Specifically, when we say there is growth discordance of greater than 20%, it indicates a notable disparity in fetal growth that could impact the health of both twins. This threshold is crucial for identifying potential complications, such as intrauterine growth restriction (IUGR) or other pathological conditions.

In the context of twin pregnancies, particularly monochorionic twins, growth discordance often raises concern for potential complications like twin-to-twin transfusion syndrome (TTTS), where one twin receives more blood flow than the other. Understanding these growth patterns helps healthcare providers monitor the twins closely, manage any arising issues, and plan appropriate interventions if necessary.

The other ranges of discordance—less than 15% or between 15-20%—are considered within normal limits, though close monitoring is always warranted in twin gestations. Saying that growth discordance is "always present in monochorionic twins" is misleading; while these twins are at higher risk for significant discordance, not all monochorionic twin pairs experience this condition. Thus, the defining criteria for growth discordance emphasizes the importance of identifying when the growth difference exceeds 20%.