Legal case against doctor for negligence of duty

My wife was undergoing treat for her pregnancy at a private clinic, the doctor herself conducted the ultrasound 3 times and ultrasound was also conducted at a private lab which have advance 3d/4d ultrasound equipments. After the delivery, it case as surprise that baby right arm was not developed beyond elbow. Total 6 times ultrasound were done (3rd month and so on) but each time they fail to diagnose it.after the delivery when i asked the doctor she said somehow it was missed, and verbally accepted her mistake, but as parents we feel cheated and now left to suffer. I feel that by not verifying, she has failed to carry t is doctor has failed and should be held accountable.