An extra chromosome can cause a variety of birth defects, depending on which chromosome is affected, causing disorders such as Down syndrome and Edwards syndrome. In order to be born with an extra chromosome, one pair of chromosomes must have an extra chromosome attached to it, called a trisomy.
There are different possible trisomies, some more common than others. For example, trisomy 18 is associated with Edwards syndrome, which causes many physical defects like organ malfunctions and webbed appendages. Another common trisomy is trisomy 21, commonly referred to as Down syndrome. Down syndrome is characterized by a flattened face, slanting eyes and unusually short arms and legs.