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. Highly accurate, prenatal genetic testing can diagnose a baby's syndrome, allowing time for the staff to plan medical interventions and the parents to prepare for the birth of the baby.
Procedures
To obtain an unborn baby's DNA chorionic villus sampling--or CVS--obtains a small piece of the placenta between 10 and 13 weeks gestation. Another procedure, amniocentesis, extracts--between weeks 15 and 16 of pregnancy--a sample of the amniotic fluid that surrounds the baby. This fluid contains the baby's cells from which DNA can be extracted. These invasive procedures carry a small risk for miscarriage and other pregnancy complications.
Sample Type
The type of sample obtained affects the accuracy of prenatal genetic testing. CVS samples are slightly less accurate than amniocentesis samples. CVS samples contain cells from both the mother and the baby. Well-trained technicians can tell the difference between tissue from the mother and tissue from the baby. This presents less of a problem when the baby is male because of the presence of the Y chromosome. With CVS there is a small chance placental DNA may be different from the baby's DNA. Since DNA from amniocentesis is directly from the baby, it is generally more reliable.
Chromosome Syndromes
Prenatal genetic testing can detect Down syndrome and other chromosomal disorders. Normally, a person will have 46 paired chromosomes, one set of which comes from the mother and the other, from the father. Sometimes a child has too many or too few chromosomes. Chromosome analysis has an accuracy rate greater than 99 percent.
Single-Gene Disorders
In a single-gene disorder--such as cystic fibrosis and sickle cell anemia--a change in a specific gene causes disease. Often, a baby must inherit a defective gene from each parent to be affected. After identification of the defective gene in both parents, prenatal genetic testing will look in the DNA of the fetus for the specific gene change found in each parent. Depending on the gene and the laboratory performing the test, accuracy runs to about 99 percent.
Considerations
Prospective parents may find it difficult to decide whether to perform prenatal genetic testing. A couple should ask questions about genetic testing and the procedures involved before agreeing to the testing. A genetic counselor or other medical provider knowledgeable in genetics can help answer these questions.
References
- Thompson & Thompson Genetics in Metabolism (7th Edition); Robert Nessbaum; 2007
- Obstetrics & Gynecology: Invasive Prenatal Testing for Aneuploidy
- Guide to Genetic Counseling; Diane Baker; 1998
