Before the beginning: the genetic risk of a couple aiming to conceive.

Disorders of genetic etiology exist in 2%-3% of live-born infants. Identifying couples with increased susceptibility for offspring with anomalies or genetic disorders is increasingly effective as a result of molecular advances. Preimplantation genetic testing (PGT) with the use of trophectoderm biopsy, 24-chromosome testing, and molecular testing have allowed wider applicability for avoiding a clinical pregnancy termination. Cell-free DNA in maternal blood is another targeted option, although invasive prenatal genetic diagnosis provides the greatest amount of genetic information. DNA-based methods to detect subtle chromosomal abnormalities are much more sensitive than traditional karyotypes and do not require cultured cells. Aneuploidy and structural chromosomal abnormalities can be readily detected with the use of small amounts of DNA, if necessary amplified, as in PGT. Novel approaches exist for detecting perturbations in single-gene disorders. Not only has the molecular basis for many monogenic disorders been elucidated, but modest costs for DNA sequencing has made testing feasible. As the number of testable genetic disorders has increased, principles underlying screening have advanced. Genetic screening for disorders of high incidence in certain ethnic groups was initiated decades ago; however, limitations exist, and reduction in live-born incidence is not infrequently small. Expanded carrier screening is now offered in panethnic fashion, extending surveillance to couples of mixed ethnicities and involving many more genetic conditions. Targeted gene panels (e.g., adult-onset cancer genes) further increase the number of genetic disorders amenable to screening, often leading to PGT.