What you describe sounds like an echogenic cardiac focus. On ultrasound, it looks like a bright spot in the muscle of one of the heart valves. In and of itself, it will not cause the baby any problems; it is not a heart defect and does appear to increase the baby's risk of having a heart defect. It is a common finding in normal babies. But, it may very mildly increase the risk that the fetus has Down Syndrome.
The first step is to see if there are any other ultrasound markers for Down Syndrome. As long as this is an isolated finding, the risk of Down Syndrome is increased only slightly — between 1.8 and 5-fold over your age-related or calculated risk (based on whatever screening test for Down Syndrome your doctor performed). As long as you are less than 35 years old and your screening tests and ultrasounds are normal, I generally do not recommend an amniocentesis. Of course, if you have already had a normal amniocentesis, this finding is nothing to worry about.