This chapter provides examples of birth defects due to chromosomal abnormalities of both sex chromosomes and autosomes and examples of birth defects due to abnormalities of both chromosome number and chromosome structure. It discusses teratology and the general categories of teratogens, and explains the concept of “critical periods in development” and the practical consequences of their existence. The degree of severity of birth defects ranges from relatively minor to absolutely devastating. Some persons with birth defects require institutionalization. A birth defect may be caused by a tiny mutation in DNA inherited from the parents or by a chromosomal defect of some sort. Examination of spontaneous abortuses has revealed that many have chromosomal abnormalities. The chapter also discusses the various methods of detection of birth defects and what might cause a woman to choose one method over the others. Chromosomal abnormalities may affect either the sex chromosomes or the autosomes.