Why do relatively few fetuses with chromosomal trisomies survive to birth?

A trisomy is a condition in which there is one more chromosome than normal, meaning that in humans there are three replicates of a particular chromosome, instead of the normal pair of two. In most of the times, trisomies lead to miscarriages. In rare cases, it can lead to live birth but it will be associated with shortened life expectancy and disabilities. In humans, autosomal chromosomes are essential to life and an abnormal increase of their number can be lethal. The fetus will not be able to develop properly and this will lead to a miscarriage.