Trisomy 13 mosaicism at prenatal diagnosis: dilemmas in interpretation.

We describe six cases of trisomy 13 mosaicism detected at prenatal diagnosis. Most level I and level II trisomy 13 mosaicism detected at prenatal diagnosis is pseudomosaicism or confined placental mosaicism. Rarely, low-level mosaicism at chorionic villus sampling or amniocentesis reflects a true fetal mosaicism. In this case, a normal phenotype is a possible, but not a certain, outcome. Genetic counselling is not straightforward.