What is the definition or description of: ultrasound-guided amniocentesis?

Amniotic sample: Needle puncture of pregnant uterus, under ultrasound guidance, to obtain an amniotic fluid sample. Diagnostic or therapeutic. Diagnostic for fetal chromosome defects such as Down syndrome or ? Neural tube defect like myelomeningocele. Rare therapeutic cases to decrease volume of amniotic fluid for xs fluid (polydramnios) or increase amniotic volume for too little fluid (oligohydramnios).