Prenatal Genetic Testing
Prenatal Genetic Testing
Genetic testing can be performed to detect chromosomal abnormalities such as Down syndrome as well as individual diseases caused by gene mutations such as Cystic Fibrosis or Tay-Sachs disease. Testing is generally categorized as being either a screening test or a diagnostic test.
Screening tests are recommended for the general population thought to be at average or low risk. They are non-invasive tests, which pose no immediate risk to you or your pregnancy. Screening tests determine whether you are at reduced risk or higher than expected risk for a given condition.
Screening tests cannot provide a definitive answer about your baby having a specific disorder or birth defect. Results of a screening test have a chance to be incorrectly positive or incorrectly negative.
Cell Free DNA Screening
This screening is available anytime after 10 weeks gestation. It typically requires a single maternal blood draw. This screen provides risk information on Down syndrome, trisomy 18, trisomy 13, and the sex chromosome disorders, such as Turner syndrome and Kleinfelter syndrome. This risk is determined through analysis of small fragments of DNA released from placental cells.
Compared to other screening tools, the cell free fetal DNA screen has a higher detection rate for these chromosome disorders with fewer false positives.
Sequential screening provides risk information for Down syndrome, trisomy 18 and open fetal defects such as spina bifida.
It is a method of combining both first trimester and second trimester screens, which are described below.
First Trimester Screening
The first trimester portion is a combination of maternal blood and fetal ultrasound, which is performed between 10 to 13 weeks gestation. It can provide a preliminary risk for Down syndrome and trisomy 18. An ultrasound measurement of the thin layer of fluid found at the back of the fetal neck is obtained.
Specific risk estimates are calculated by taking into account the ultrasound measurement, your age and the levels of two biochemical markers in your blood that are produced during pregnancy.
Second Trimester Screening (AFP4)
This second trimester blood draw is collected between 15 and 21 weeks. Second trimester risk levels are calculated by taking into account your age and the levels of four biochemical markers being produced during the pregnancy.
The first trimester and second trimester portions of the screen are combined to refine and provide risk information for Down syndrome, trisomy 18 and open fetal defects, such as spina bifida.
If either the first trimester or the second trimester portion of screening provides an increased risk estimate, further genetic counseling would be available to discuss additional screening or diagnostic testing options.
Carrier screening is available to help couples learn more about their risks of having a child with certain genetic conditions, like cystic fibrosis, Tay-Sachs disease, or sickle cell anemia. Expanded carrier screening is available to determine risk for approximately 80 genetic conditions. In certain ethnic groups some genetic conditions are more common. Many couples at risk of having a child with a genetic disorder have no prior family history.
Diagnostic tests are offered to patients believed to be at elevated risk for certain birth defects and other couples who would benefit from more direct testing of the pregnancy. Diagnostic tests determine whether or not the condition you are testing for is present. The tests are invasive, which may cause a miscarriage.
Chorionic Villus Sampling (CVS) is a diagnostic test which involves sampling cells from the placenta. It is performed between 10 and 13 weeks gestation. Using ultrasound guidance, either a needle is placed through the abdomen into the placenta or a catheter is guided through the cervix to obtain a biopsy of the placenta. The placental cells can be analyzed for Down syndrome or other genetic diseases. Standard results return in 7-10 days.
Amniocentesis is a diagnostic test which analyzes the amniotic fluid surrounding the fetus. It is performed after 15 weeks gestation. Using ultrasound guidance, a needle is placed through the abdominal wall into the uterus. Approximately 20ml of amniotic fluid is withdrawn. The fetal cells in the fluid can be analyzed for Down syndrome or other genetic diseases. It can also screen for certain birth defects, like spina bifida. Standard results return in 10-14 days.