THICKNESS OF INTER-TWIN MEMBRANE

2mm thickness is used to differentiate between thick and thin.

• Positive predictive value
• 95% for dichorionic twins.
• 82% for monochorionic diamniotic twins (1).

Thickness is subjective, and is good but not optimal in differentiating monochorionic from dichorionic twins. Counting the layers of the intertwin membrane is difficult but is getting easier with improvements in ultrasound technology (2).

Dichorionic	2 layers of amnion and 2 layers of chorion. * Thicker. * More reflective.
Monochorionic Diamniotic	2 layers of amnion. * Thinner. * Less reflective.

REFERENCES

1. Winn HN, Gabrielli S, Reece EA et.al. Ultrasonographic criteria for the prenatal diagnosis of placental chorionicity in twin gestations. Am J Obstet Gynecol 1989;161:1540-1542.
2. Stagiannis KD, Sepulveda W, Southwell D et.al. Ultrasonographic measurement of the dividing membrane in twin pregnancy during the second and third trimesters: a reproducibility study. Am J Obstet Gynecol 1995;173:1546-1550.