Prenatal Diagnosis Also known as a sonogram, an ultrasound reflects sound waves off of the fetus and then is shown on a special computer screen. This test is not very clear and physical features such as limb, heart, brain, and spine abnormalities can't be seen until 16-18 weeks of pregnancy. The ultrasound mostly only detects things of structural and/or physical nature. Things like mental retardation, blindness, and deafness can't be detected by an ultrasound nor can it detect chromosome abnormalities such as down syndrome. Ultrasound may lead to recommendation for further diagnosis. There is no known risk to the mother or the fetus from an ultra sound. Chorionic villus sampling Used