C L I N I C A L C O R R E L A T E S - Birth Defects and Spontaneous Abortions:

C L I N I C A L C O R R E L A T E S
Birth Defects and Spontaneous Abortions:
Chromosomal and Genetic Factors
Chromosomal abnormalities, which may be numerical or structural, are
important causes of birth defects and spontaneous abortions. It is estimated
that 50% of conceptions end in spontaneous abortion and that 50% of these


Figure 1.4 Events occurring during the first and second maturation divisions. A. The
primitive female germ cell (primary oocyte) produces only one mature gamete, the mature
oocyte. B. The primitive male germ cell (primary spermatocyte) produces four spermatids,
all of which develop into spermatozoa.
abortuses have major chromosomal abnormalities. Thus approximately 25%
of conceptuses have a major chromosomal defect. The most common chromosomal
abnormalities in abortuses are 45,X (Turner syndrome), triploidy,
and trisomy 16. Chromosomal abnormalities account for 7% of major birth
defects, and gene mutations account for an additional 8%.
Numerical Abnormalities
The normal human somatic cell contains 46 chromosomes; the normal gamete
contains 23. Normal somatic cells are diploid, or 2n; normal gametes
are haploid, or n. Euploid refers to any exact multiple of n, e.g., diploid or
triploid. Aneuploid refers to any chromosome number that is not euploid; it is
usually applied when an extra chromosome is present (trisomy) or when one
is missing (monosomy). Abnormalities in chromosome number may originate
during meiotic or mitotic divisions. In meiosis, two members of a pair
of homologous chromosomes normally separate during the first meiotic division
so that each daughter cell receives one member of each pair (Fig. 1.5A).
Sometimes, however, separation does not occur (nondisjunction), and both
members of a pair move into one cell (Fig. 1.5, B and C ). As a result of
nondisjunction of the chromosomes, one cell receives 24 chromosomes,
and the other receives 22 instead of the normal 23. When, at fertilization,
a gamete having 23 chromosomes fuses with a gamete having 24 or 22 chromosomes, the result is an individual with either 47 chromosomes
(trisomy) or 45 chromosomes (monosomy). Nondisjunction, which occurs
during either the first or the second meiotic division of the germ cells, may
involve the autosomes or sex chromosomes. In women, the incidence of
chromosomal abnormalities, including nondisjunction, increases with age,
especially at 35 years and older.