Genetic testing can be performed to detect chromosomal anomalies 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 low risk. They are non-invasive tests, which pose no immediate risk to you or your pregnancy. Screening tests determine whether or not you are at normal or higher than expected risk for a given condition. Screening tests do not determine for certain whether your baby will have a specific problem.
First Trimester Screening
First trimester screening is a combination of maternal blood and fetal ultrasound, which is used to screen for Down syndrome and Trisomy 18. Specific risk levels for these conditions are calculated by taking into account your age and the levels of two biochemical markers which appear in your blood. An ultrasound measurement of the thin layer of fluid found at the back of the fetal neck is also used. Compared to second trimester screening, first trimester screening has a better detection rate with fewer false positives; however open fetal defects such as spina bifida are not tested. Since this screening is not yet widely available, please check your insurance coverage.
Second Trimester Screening
Maternal serum screening is the standard screening test for Down syndrome, Trisomy 18 and open fetal defects such as spina bifida. Specific risk levels for these conditions are calculated by taking into account your age and the levels of four hormones being produced by the pregnancy, which appear in your blood. This test is performed between 15 and 21+ weeks.
Sequential screening is the newest screening option 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.
If you are African American, Jewish, French Canadian, Cajun or have a family history of particular genetic diseases such as Cystic Fibrosis, other screening tests may be appropriate.
Diagnostic tests are offered to patients believed to be at elevated risk who would benefit from more direct testing. Diagnostic tests determine whether or not the condition you are testing for is present. They are invasive tests, which test the pregnancy directly and 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 12 weeks gestation. Using ultrasound guidance, a needle is placed via the vagina or abdomen into the placenta.
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 fluid can be analyzed to determine if a fetus has an open fetal defect such as spina bifida. The fetal cells in the fluid can be analyzed for Down syndrome or other genetic diseases.