What is non invasive prenatal diagnosis?

Ultrasound. Noninvasive means test being performed does not enter the body i.e. Down the throat/airway, up the vaginal canal, anal canal, or piercing through the skin/tissue such as entering blood vessels, abdominal cavity etc... An ultrasound done without having to do above is noninvasive--such as checking gallbladder for stones etc...
Without risk to fetu. New technology instead of getting a tissue sample from the fetus or amniotic fluid by using needles, just from mother's blood fetal blood cells can be extracted. These fetal cells then are tested for genetic conditions. Common noninvasive studies are fetal ultrasounds or mri's; images of fetal body parts can lead to a diagnosis. Maternal serum tests are only screening studies, not to diagnose.