The Number of Yolk Sacs Does Not Always Equal the Number of Chorions in Multiple Gestations

The number of yolk sacs is not an absolutely accurate indicator of chorionicity in multiple gestations. 1

Relationship Between Yolk Sacs and Chorionicity

In the first trimester, determining chorionicity is most accurate by evaluating the number of gestational sacs, which equals the number of chorions, with nearly 100% accuracy 1
While it has been suggested that monoamniotic twin pregnancies have a single yolk sac, this is not always the case, making yolk sac counting an unreliable method for determining chorionicity 1
Studies have shown discrepancies between the number of yolk sacs and amnionicity in approximately 9.3% of monochorionic gestations 2

Cases have been documented where monochorionic-monoamniotic twin pregnancies had two yolk sacs visible on ultrasound examination 3, 4
Conversely, some monochorionic-diamniotic twin pregnancies have been found to have only a single visible yolk sac 2
In one study, 4 cases with 2 yolk sacs were confirmed as monoamniotic pregnancies, and 4 monochorionic twin pregnancies showing a single yolk sac were diagnosed as diamniotic twin pregnancies 2

The most accurate determination of chorionicity is achieved in the first trimester by counting gestational sacs 1
For amnionicity determination, careful evaluation of the intertwin membrane via transvaginal ultrasound after 10 weeks gestation is more reliable than counting yolk sacs 1, 5
When amnionicity remains uncertain, confirmation can be achieved by demonstrating umbilical cord entanglement using color or pulsed wave Doppler to identify two different heart rates 1
After 10 weeks, additional features helpful for determining chorionicity include:
- Number of placentas 1
- Lambda or twin peak sign (seen in dichorionic gestations) versus T-sign (seen in monochorionic gestations) 1
- Dividing membrane thickness 1

Accurate determination of chorionicity and amnionicity is essential for proper risk stratification and management of multiple gestations 6, 5
Monochorionic-monoamniotic twin pregnancies carry significantly higher risks and should be referred to a tertiary center for specialized care 1, 6
Failing to correctly determine chorionicity can lead to missed opportunities for early intervention in high-risk pregnancies 5
When chorionicity cannot be determined with certainty on transabdominal scan, a transvaginal scan should be performed 1

Relying solely on yolk sac number to determine chorionicity or amnionicity can lead to misclassification 2, 3
Assuming absence of an intertwin membrane indicates monoamnionicity, as the amnion may not yet be visible at early gestational ages 5
Using a single feature rather than a combination of sonographic findings to determine chorionicity 1