More than 50 years ago, Hibbard and Smithells (1965) postulated that abnormal folate metabolism was responsible for many NTDs. For a woman with a prior afected child, the recurrence risk of 3 to 5 percent is decreased by at least 70 percent-and potentially by as much as 85 to 90 percent-with periconceptional oral folic acid supplementation at a dosage of 4 mg/d (Grosse, 2007; MRC Vitamin Study Research Group, 1991). However, most NTD cases do not occur in the setting of maternal folic acid deiciency, and it has become clear that the gene-nutrient interactions underlying folate-responsive NTDs are complex. he NTD risk may be afected by genetic variation in folate transport or accumulation, impaired folate utilization via secondary nutrient deficiencies such as vitamin BI2 or choline deficiency, and genetic variation in activity of folate-dependent metabolic enzymes (Beaudin, 2009).
All pregnant women should have the option of prenatal aneuploidy screening and prenatal genetic diagnosis (American College of Obstetricians and Gynecologists, 2016b). Aneuploidy screening may be performed with serum analyte-based screening or with a DNA-based screen, namely, cell-free DNA found in the maternal circulation. Prenatal genetic screening of the parents also aids carrier status determination in at-risk individuals (Chap. 14, p. 288).
For prenatal genetic diagnosis, the most commonly used tests are cytogenetic analysis (karyotyping), luorescence in situ hybridization (FISH), and chromosomal microarray analysis. Testing may be performed on amnionic fluid or chorionic villi. In selected circumstances, whole genome or whole exome sequencing may be considered, but these are not recommended for routine use. To diagnose a specific disease whose genetic basis is known, DNA-based tests are often employed, typically using polymerase chain reaction (PCR) for rapid ampliication of DNA sequences.
Karyotype analysis is commonly performed to test for chromosomal abnormalities. Any tissue containing dividing cells or cells that can be stimulated to divide is suitable for cytogenetic analysis. Karyotyping detects numerical abnormalities, that is, aneuploidy. It also identiies balanced or unbalanced structural rearrangements of at least 5 to 10 megabases in size. Karyotyping has diagnostic accuracy exceeding 99 percent.
The dividing cells are arrested in metaphase, and their chromosomes are stained to reveal light and dark bands. The most commonly used technique is Giemsa staining, which yields the G-bands shown in Figure 13-3. Each chromosome has a unique banding pattern that permits its identiication and detection of deleted, duplicated, or rearranged segments. he accuracy of cytogenetic analysis rises with the number of bands produced. High-resolution metaphase banding routinely yields 450 to 550 visible bands per haploid chromosome set. Banding of prophase chromosomes generally yields 850 bands.
Because only dividing cells can be evaluated, the rapidity with which results are obtained correlates with the rapidity of cell growth in culture. Amnionic fluid, which contains epithelial cells, gastrointestinal mucosal cells, and amniocytes, usually yields results in 7 to 10 days. Fetal blood cells may provide results in 36 to 48 hours but are rarely needed (Chap. 14, p. 294). If fetal skin fibroblasts are evaluated postmortem, stimulation of cell growth can be more diicult, and cytogenetic analysis may take 2 to 3 weeks (Chap. 35, p. 647).