Investigation of choice for diagnosing downs syndrome at 15 weeks of gestation in a mother with a history of delivering a down syndrome baby is which of the following:

Amniocentesis is the investigation of choice for the diagnosis of downs syndrome. It is done between 14-16 weeks of gestation. It is performed in the context of prior history of a child with downs syndrome, advanced maternal age, abnormal triple or quad assay. Ref: Harrison's Principles of Internal Medicine, 17th Edition, Page 409 ; Textbook of Obstetrics By D.C.Dutta, 6th Edition, Page 107