CHROMOSOMAL ABERRATIONS
Aneuploidy refers to numeric chromosome abnormalities in which cells
contain other than 2 complete sets of 23 chromosomes. This usually occurs
because of nondisjunction.
The most common aneuploidy is trisomy, the presence of an extra
chromosome.
Most autosomal trisomies result in spontaneous abortion.
The most common trisomy in first-trimester losses is trisomy 16.
The most common trisomy at term is trisomy 21.
Polyploidy refers to numeric chromosome abnormalities in which cells
contain complete sets of extra chromosomes. The most common polyploidy
is triploidy with 69 chromosomes, followed by tetraploidy with 92
chromosomes. An example of triploidy is an incomplete molar pregnancy,
which occurs from fertilization of an egg by two sperm.
Structural alteration refers to a condition in which chromosomal material is
deleted, gained, or rearranged. It can involve single or multiple chromosomes.
An example of a chromosomal deletion is del (5p) or cri du chat syndrome,
which is a deletion of the short arm of chromosome 5.
Mosaicism refers to the presence of ≥2 cytogenetically distinct cell lines in
the same individual. Mosaicism can involve the placentae, the fetus, or both.
Gonadal mosaicism can result in premature ovarian failure and predispose
the gonad to malignancy.