Cytogenetics and molecular cytogenetics.

Obstetricians are increasingly called upon to deal with the pregnant woman’s risk for genetic and cytogenetic conditions. Chromosome disorders are a major category of genetic disease. They account for a large proportion of reproductive wastage, congenital malformations, and mental retardation and are important in the pathogenesis of malignancy. Specific chromosome abnormalities are responsible for >100 identifiable syndromes that are collectively more common than all mendelian single gene disorders together. The incidence of abnormalities of the autosomes and sex chromosomes is approximately the same. The effect of these abnormalities on the child will depend on the type of abnormality. Abnormalities are mild in disorders of most of the sex chromosomes. Unbalanced autosomal abnormalities cause disorders with multiple congenital malformations, almost invariably associated with mental retardation. Cytogenetic disorders are present in almost 1% of live births, in about 2% of pregnancies in women older than 35 years of age who undergo prenatal diagnosis, in half of all spontaneous firsttrimester abortions, and in 6% of all stillbirths. Thus, an understanding of cytogenetics and chromosomal disorders is important in daily practice. As there is no effective therapy for the chromosomal disorders (excepting the sex chromosome abnormalities), detection during the fetal state and prevention through selective pregnancy management is the only successful approach. The most common indication for prenatal genetic studies is to search for a chromosomal abnormality by screening the general pregnant population for evidence of increase in risk and then performing (usually) an invasive diagnostic study. The historical indicator of risk was “advanced maternal age.” The chance of carrying a fetus with a chromosomal disorder increases with the age of the mother from approximately 1:250 at age 35 to 1:65 at 40 and 1:20 at 45 years of age. This indicator is now supplemented by a panel of maternal serum biochemical tests and ultrasound measurement of fetal features associated with chromosomal disorders. In many areas, all pregnant women are offered such tests and their risk is then managed by the obstetrician. Correspondence: Laird Jackson, MD, Department of Obstetrics and Gynecology, MCP Hahnemann University, 245 N. 15th Street, MS 495, Philadelphia, PA 19107. Email: lgj25@drexel.edu PROD. # GRF20338