21 weeks pregnant, tested negative for Down syndrome, and found a calcuim deposit in baby's heart. What could it mean? Help please.

Ask your obstetrician. Who would help you better, my take on this, is that it is not uncommon, found in about 10% of scans of normal babies, other parameters factor in as rate of growth, other appearances on the scan, your blood tests and others, please confer with your obstetrician who will be able to address your concerns better.
Benign. this is called echogenic chordae tendinae or papillary muscles, the structures which pull open the mitral and tricuspid valves. It is also called an intracsrdiac focus. This ultrasound finding turns out to have no significance whatsoever. A high frequency of fetuses have this finding--it is NOT calcium deposition. .