A genetic accident. Human cells are supposed to have 23 chromosomes from father's sperm that pair up with 23 chromosomes from mother's egg. If either donation has an extra chromosome because of a genetic error, it is passed along to the fetus most common extra chromosomes in live births are on 13, 18 and 21. They cause unusual physical features, intellectual disability and, sometimes, organ malformations.
Error in formation. During formation of either egg or sperm (gametes), the chromosome (c) pairs that make up the normal cell separate and each gamete is supposed to have 1/2 of a pair. If any c pair does not separate, three rather than a pair of that c will form as egg & sperm join. Having an extra c in every cell of the fetus confuses the fetal development. Miscarriage or birth defects often occur.
Depends. Some extra chromosomes are incompatible with life. Some extra chromosomes may be associated with mental retardation and congenital deformities. Some extra chromosomes may be associated with behavioral problems. Some extra chromosomes may not cause any noticeable problems.