Prenatal diagnosis of trisomy 18 as true mosaicism by three-dimensional ultrasonography: a case report.

Trisomy of chromosome 18 is the second most common autosomal trisomy, occurring in approximately 1:7,000 live births. Its prenatal diagnosis through abnormal findings in ultrasound with later analysis of fetal karyotype is important for a definition of the prognosis and counseling of the patients. We describe a case of trisomy 18 as true mosaicism diagnosed through amniocentesis in the second trimester of pregnancy, associated to the presence of multiple fetal phenotypic alterations. We focus on the importance of fetal morphological study through three-dimensional ultrasonography, which was highly important for clearly showing the fetus' structural alterations, helping parents to understand better the pathology and allowing them to reason about the continuity of the gestation.