Polar bodies serve to eliminate one half of the diploid chromosome set in meiotic division in the egg, leaving behind a haploid cell. That means that 23 pairs (46) of chromosomes are going to be separated equally, 23 for the polar body and 23 for the egg cell. In case of trisomy 18, the division is not equal. Instead, the egg cell receives 23 +1 chromosomes and the polar body receives 23 - 1= 22 chromosomes.
That chromosome that was misplaced is the chromosome 18.