How does an extra chromosome set cause big problems for the baby?

Multiorgan effects.: Aneuploidy (i.e. One extra or missing chromosome) is a group of major genetic disorders that can affect all organ systems, more commonly the heart, brain and kidneys. Examples include Down syndrome (mongolism) and edwards' syndrome (usually lethal). The exact mechanism by which the chromosomal aberrration leads to these effects is not entirely clear but may have to do with gene silencing.