When prenatal screening identifies features of a genetic syndrome or there is a family history of genetic disease, a woman's health care provider may suggest prenatal genetic testing. Baby's DNA, obtained by chorionic villus sampling--commonly referred to as CVS--or amniocentesis is tested to identify if the baby is affected with a genetic syndrome. Diagnosis before birth allows time to establish medical interventions and the parents to prepare for the baby's birth. Both procedures are invasive and carry a small risk for miscarriage and other pregnancy complications.
Chorionic Villus Sampling
Chorionic villus sampling, or CVS, obtains a small piece of the placenta. Earlier than an amniocentesis, CVS is performed between 10 weeks and 13 weeks of pregnancy. Placental cells obtained contain the same DNA likely present in the baby. Drawbacks to this procedure include the fact that there is a higher chance the sample may be contaminated by the mother's DNA and, in rare cases, the placental DNA can be different from the baby's DNA. When complications occur, an amniocentesis is often performed to clarify testing results from a CVS sample.
Amniocentesis
An amniocentesis is typically performed between the 15th and 16th week of pregnancy. A sample of the amniotic fluid containing the baby's cells is extracted. DNA obtained from these cells can be tested for different genetic conditions. This procedure is generally more accurate as it is performed directly on the baby's DNA, according to "Thompson & Thompson Genetics in Metabolism (7th Edition)."
Karyotype
Prenatal genetic testing can detect Down syndrome and other chromosomal disorders. Chromosomes are structures of DNA and proteins in which the genes are located. Normally, there are 23 paired chromosomes--46 total. One set is inherited from the mother and the other set from the father. Sometimes, mistakes occur when the chromosomes are inherited resulting in a child having too many or too few chromosomes. Other times, smaller losses or gains of DNA occur. A test called a karyotype looks at the chromosomes to see if they are correct in number, structure and amount of DNA.
Prenatal Microarray
A prenatal microarray is similar to karyotype, but is much more sensitive. It may be ordered if the baby seems to have a chromosomal disorder, but the karyotype was normal. A microarray detects very small gains and losses in DNA and evaluates for more genetic syndromes than a karyotype. A microarray is so powerful, sometimes the significance of the results are unknown. It is for this reason that a prenatal microarray is not offered in every pregnancy.
Testing for Single gene disorders
Single gene disorders--such as cystic fibrosis or sickle cell anemia--are syndromes in which a change in a specific gene is responsible for disease. This testing is generally done when a family history of a specific syndrome is present, according to "Thompson & Thompson Genetics in Metabolism (7th Edition)." Genetic testing can determine if the baby is affected or not. Testing on parents must occur first to identify the type of change present. Once a familial change has been found, prenatal genetic testing can be done, looking for the specific gene change found in a parent.
References
- "Thompson & Thompson Genetics in Metabolism (7th Edition)"; Robert Nessbaum; 2007
- "Obstetrics & Gynecology"; ACOG Practice Bulletin No. 88: Invasive Prenatal Testing for Aneuploidy; December 2007
