It is possible to test a baby's DNA before birth. Known as prenatal genetic testing, this is generally performed when there is a family history of a genetic syndrome or features of a genetic syndrome have been during the pregnancy. Prenatal genetic testing differs from prenatal screening, which is performed on the mother's blood. The baby's DNA is obtained by an amniocentsis in which amniotic fluid is drawn from the amniotic sac or by chorionic villus sampling (CVS). Another method is to obtain a tiny sample of the placenta.
Prenatal Screening
In prenatal screening, blood drawn from the mother is analyzed for proteins in the mother's blood during pregnancy. If these protein levels are abnormally high, abnormally low, or show a specific pattern, there is a higher chance the baby may be affected with certain genetic syndromes, such as Down Syndrome. An "abnormal screening test" means the baby is more likely to be affected with a genetic syndrome, but it is unknown if the baby has the syndrome. Prenatal genetic testing can be performed to look at the baby's DNA and determine if he is affected with the syndrome.
Function
There are several reasons why a doctor may offer prenatal genetic testing to a family. The most common reasons for prenatal genetic testing including that the mother is 35 years or older, there is a family history of a genetic disorder, or an abnormal finding has been identified by maternal serum screening or ultrasound.
Methods
DNA for prenatal genetic testing is obtained through the use of invasive procedures. An amniocentesis is a procedure in which a needle is inserted into the amniotic sac to remove a sample of amniotic fluid. Contained in the amniotic fluid are cells from the baby. DNA can be obtained from these cells. CVS is a procedure in which a small piece of the placenta is removed. The DNA in the placenta is generally the same as the baby's DNA. Since these tests are invasive, there is a small risk for miscarriage. The pros and cons of these procedures should be thoroughly discussed with a doctor.
Conditions
A doctor may recommend prenatal diagnosis based on an abnormal maternal serum screen or ultrasound. Generally, chromosome testing, also called a karyotype, will be ordered. Chromosomes are structures of DNA and proteins in which the genes are located. Normally, there are 46 chromosomes and they are paired. 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. Common syndromes that are identified prenatally include Down syndrome (also called Trisomy 21), Edwards syndrome (Trisomy 18), Patau syndrome (Trisomy 13) and Turner syndrome.
Family History
If there is a family history of a genetic disease, prenatal diagnosis can be done to determine if the baby also is affected. Often, this entails looking at a specific gene that is known to cause disease. Common syndromes that may require this type genetic testing are cystic fibrosis or sickle cell anemia. Prenatal genetic testing is most accurate when a previous family member has been identified as having a genetic change, or mutation, for the genetic disease in the family.
Considerations
The decision to undergo prenatal testing can be difficult. A couple should ask questions about genetic testing and the procedures involved before agreeing to the testing. A genetic counselor or other medical provider can help answer these questions. Some families choose not to have genetic testing because they do not want to risk miscarriage from an invasive testing or because it would not change the outcome of the pregnancy. Other families decide to undergo testing because a diagnosis may allow them to prepare for the birth of an affected child. Many families make these decisions based on their own personal beliefs, ethics and culture.
References
- Thompson & Thompson Genetics in Metabolism (7th Edition); Robert Nessbaum; 2007
- American Congress of Obstetricians & Gynecologists; Routine Tests in Pregnancy
- PlubMed: ACOG Practice Bulletin No. 77: Screening for Fetal Chromosomal Abnormalities
- Guide to Genetic Counseling; Diane Baker; 1998
