Genetic amniocentesis was made possible in the 1970s after it was shown that fetal cells could be cultured and grown. Fetal epithelial (skin) and other cells are sloughed from the fetus and are present in the amniotic fluid. By getting a sample of the fluid, there is no need to sample the fetus directly. Fetal cells are cultured and grown because there are relatively few cells initially. After culturing the cells, a process of karyotypic banding is performed. The DNA is stained with a special stain (Giemsa stain) during a critical part of cell division. Using a high powered microscope, the DNA can then be visualized and analyzed. A full karyotype analysis looks for not only major chromsome abnormalities- such as abnormal number of chromosomes- but also much smaller abnormalities.
CHORIONIC VILLUS SAMPLING (CVS)
Chorionic villus sampling (CVS) requires sampling of the placental
tissue by either placing a needle through the skin (percutaneous) or by
inserting a flexible catheter through the vagina and cervix and into the
placenta. This procedure is based on the fact that in nearly all cases,
the fetal and placental tissue is the same, so that sampling of the
placenta is representative of the fetus. The primary advantage of CVS
is the ability to detect problems sooner than with amniocentesis. With
CVS, results are available at 10 to 12 weeks of pregnancy. CVS can be
used to determine virtually all disorders that can be diagnosed by
amniocentesis with the exception of neural tube defects (spina bifida).
CVS is the primary tool used to diagnose fetal cytogenetic, molecular
and biomedical disorders, such as cystic fibrosis, hemoglobinopathies,
hemophilia, sickle-cell disease and Rh typing.
PERCUTANEOUS UMBILICAL VEIN SAMPLING
Umbilical vein sampling requires placing a needle into the umbilical vein and obtaining fetal blood for analysis. It is a higher risk procedure. Because of the small size of the umbilical vein, this procedure is usually performed after 20 weeks.