[Determination by ultrasound of chorionicity in twin pregnancy]

Twins, specially those of monochorionic and monoamniotic pregnancies, are exposed to many perinatal risks and complications. The objective of this study was to evaluate the usefulness of ultrasonographic determination of the chorionicity of human placental by counting the number of layers in amniotic membranes. Thirty-eight patients with twin pregnancies were studied prospectively. The ultrasonographic evaluation of membranes layers was made only once, between the weeks and 16 and 30 of pregnancy. When two layers were identified, the placentation was determined as monochorionic, and when four layers were seen, the diagnosis of bichorionic placentation was made. The type of chorionicity was confirmed by histologic study of the placenta. With ultrasound, the chorionicity was correctly determined in 36 out of 38 cases, for a total predictive value of 94.6%. The capacity for diagnosing bichorionic (4 layers) placentation was 100% (22/22) and 87.6% (14/16) for monochorionic (2 layers) placentation. The ultrasonographic evaluation of the amniotic membranes number is an efficient method to recognize the chorionicity of placenta and constitutes an useful and simple method giving important information in perinatal prognosis.