GENETIC AMNIOCENTESIS

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.

  • Usually performed 14-20 weeks
  • Risk of fetal death estimated as 1:200.  This risk is not due to hurting the fetus at the time of the procedure, but rather because the hole made by the needle may not seal properly, allowing amniotic fluid to escape.

 

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.

CVS slightly increases the chance of miscarriage (less than 1%). Other minor complications, such as vaginal bleeding (spotting) or cramping, occur more frequently after CVS than after amniocentesis.

  • Usually performed 10-12 weeks

  • Risk of fetal death estimated as 1:100 to 1:200.

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.