Structural chromosomal abnormalities include translocations, inversions, deletions, ring chromosomes, isochromosomes, intrachromosomal duplications, insertions, and dicentric chromosomes (Figures 6.1 and 6.2).

Inversions involve two breaks within a single chromosome with the intervening segment inverted.

Deletions are the result of a distal break or two internal breaks on the chromosome, allowing portions of chromosomal material to be lost. When two breaks occur, a ring chromosome can be formed.

Isochromosomes are usually mirror image chromosomes that result from single breaks close to the centromere, thus eliminating the whole of a long or short arm.

Chromosomeabnormalities represent the largest categoryof causes of deaths in humans (Table 6.1). Abortuses that have reached a 2-week stage of development are estimated to have a 38% rate of chromosome abnormalities. In liveborn infants, the rate of chromosome defects is 1:200.

In stillbirths (≥20 gestational weeks), the rate of chromosome abnormalities is between 6.6% and 11.7%. In fetuses between 9 and 20 weeks, the rate of chromosome abnormalities is around 7%. The average rate of chromosome defects of spontaneously aborted embryos earlier than 9 weeks is about 60% (59.1%) and above 60% in abortuses of a developmental age less than 7 weeks. It is conceivable to the rate of chromosome abnormalities before 2 weeks (i.e., between fertilization and the early previllous stages shortly after implantation) is 82%.

Fetuses with abnormal chromosomes are encountered after termination of Pregnancy for prenatal detection of a chromosome anomaly; after termination of pregnancy when major fetal anomaly or intrauterine fetal death has been diagnosed by ultrasound examination; and, less commonly, in a second trimester spontaneous abortus.