When the mother has been identified as a carrier of an X-linked mutation and a specific test for the fetus is unavailable, fetal sex may be determined with the option to abort males, although half of them will be normal. DNA analysis can identify the affected male for many X-linked disorders. Karyotypes are obtained from both cultures, although those based on the long-term specimen are more accurate. Most, if not all, of the enzymes in cultured amniotic fluid cells can be measured in extracts of chorionic villi or cultured villus cells, and DNA can be extracted for molecular genetic studies. The primary advantage of CVS is that results are available much earlier in pregnancy.
Normal results decrease parental anxiety earlier.
There are two CVS approaches. Transcervical CVS is performed at 10 to 13 wk gestation. Transabdominal CVS may be performed from the 10th wk of gestation to term. It is contraindicated if the bowel or bladder interferes with the needle path or if the skin in the area of needle insertion is infected. Loss rates for transabdominal CVS performed after 12 wk gestation have not been extensively evaluated.