Introduction Structural malformations, many of which can be diagnosed antenatally, are present in approximately 2%–3% of live births. Fetuses with structural malformations are at increased risk for an underlying genetic disorder, even in the setting of a normal karyotype. The…
Introduction Soft ultrasound markers were initially described as a screening method for trisomy 21 to improve the detection rate over that based on age-related risk alone. Soft markers are not structural abnormalities; rather, they are minor ultrasound findings identified in…
Overview Cell-free DNA (cfDNA) screening has been rapidly introduced into prenatal care since it became clinically available in 2011. cfDNA screening can detect more than 99% of cases of trisomy 21 and also has high sensitivity and specificity for the…
What Is the History of Serum Screening? Prenatal screening for aneuploidy has evolved dramatically over a short period of time. The purpose of prenatal screening for aneuploidy is to identify women who are at an increased risk for the most…
Introduction Prenatal genetic carrier screening has evolved over a period of about 25 years. Advances in technology and health policy have ushered this evolution. With these changes, new terminology has appeared, whereas old terms and their definitions have evolved. It…
Introduction In 2003, the sequencing of the human genome propelled the field of genetics in two important directions. Firstly, the concept of “genomics” emerged, which includes both the DNA sequence itself (the blueprint letters that direct genetic contributions to health,…
What Are Structural Chromosome Rearrangements and How Frequently Are They Seen in the General Population? As part of cytogenetic testing, the structure of each chromosome is assessed for (1) evidence of losses or gains of material; (2) internal rearrangements within…
What Is Cytogenetics? Cytogenetics is the study of chromosomes and the clinical application of chromosome analysis for diagnostic purposes. This chapter will deal with traditional cytogenetics, which involves examination of metaphase chromosomes. The normal euploid or diploid chromosome constitution in…
Overview Genetic counseling is a process of evaluating family history and medical information to advise patients of the risk of a genetic condition. This can have both a diagnostic and supportive element. The National Society of Genetic Counselors officially defines…
Introduction Mendel wrote in 1865 that when crossing plants, “numerous experiments have demonstrated that the common characters are transmitted unchanged to the hybrids and their progeny.” Mendel described that “common characters,” or genes, are passed unchanged from the parent to…