Mosaicism

Mosaicism for a marker (small unidentified) chromosome
carries a much smaller risk of causing mental retardation if
familial, and therefore the parents need to be investigated
before advice can be given. Chromosomal mosaicism detected
in chorionic villus samples often reflects an abnormality
confined to placental tissue that does not affect the fetus.
Further analysis with amniocentesis or fetal blood sampling may
be indicated together with detailed ultrasound scanning.