Prenatal Screening and Genetic Testing

Some fetal problems can be identified during pregnancy and the following narrative briefly describes some of the possible tests.  For specific indications, such as an unusual syndrome or condition, we will usually refer you for genetic counseling.  However, if you would like a consultation regarding any aspect of prenatal diagnosis or medication/environmental exposure, please discuss it with your doctor.  An appointment with a Genetic Counselor can easily be arranged.

1. AMNIOCENTESIS:   an ultrasound is used to guide the insertion of a needle through the abdomen, uterus, membranes, and sometimes placenta, into the fluid filled sac surrounding the fetus so that a sample of the fluid (about a tablespoon full) may be withdrawn and tested.  For genetic testing, it is usually performed between the 15th and 18th menstrual weeks.  The fluid contains fetal cells that are examined for the genetic or chromosome makeup of the fetus to detect problems such as Down syndrome (Trisomy 21).  Results are generally available in about 7-10 days.  Amniotic fluid is also tested for alpha-fetoprotein level that may be elevated in the presence of a neural tube defect, which is an abnormal development of the skull or spinal column.  Lastly, amniocentesis may be necessary, particularly for diabetic patients, to confirm the maturity of the baby's lungs just prior to delivery.
   
   While bleeding, infections, pregnancy loss, rupture of membranes, preterm labor, and other technical problems are among the possible complications of this procedure, they occur uncommonly - about 1 out of 300-500 procedures.
2. CHORIONIC VILLUS SAMPLING (CVS):   is done earlier in pregnancy than amniocentesis, usually during the 10th through the 14th week.  Either a small catheter is placed through the vagina and cervix into the uterus or a needle is inserted through the lower abdomen and uterus to obtain a small sample of the placental tissue to study the baby's chromosomes.
   
   Similarly, bleeding, infection, pregnancy loss occur with about the same frequency as amniocentesis.  Rarely, failure to obtain placental tissue or other technical problems can occur with this method. Learn More
3. MATERNAL SERUM ALPHA-FETOPROTEIN (MSAFP):   is measured in a sample of mother's blood taken between the 15th and 20th week.  This protein from the fetus enters both the amniotic fluid and the mother’s blood.  Maternal blood levels may be high in the presence of a neural tube defect such as when the skull or spinal column does not develop normally; alternatively, it may be elevated when there are twins or the dates are incorrect.
   
   Studies indicated that maternal blood levels of AFP were abnormally low with some chromosome abnormalities such as Down syndrome or Trisomy 21.  Therefore, the   “Multiple Marker or Quad Test" was developed to more accurately determine the risk of delivering a genetically abnormal child.  However, none of these tests provides a precise answer.  False positive results (about 5% of tests) may occur that suggest an increased risk when the fetus is perfectly normal.  Alternatively, a level that is considered normal may not identify a problem with the baby - a false negative test.
   
   Abnormal results usually necessitate additional testing that may include repeat blood testing, a sonogram to confirm gestational age, or amniocentesis.  If you have an abnormal test, we will call to discuss the results and schedule you for an appointment to review your particular circumstance.
4. TAY SACHS DISEASE:   is an inherited genetic disorder that causes destruction of the nervous system and mental retardation.  It is caused by the lack of an enzyme known as hexosaminidase-A.  Testing their blood for levels of Hex-A can identify Tay Sachs carriers.  The carrier frequency is 1 of 30 in people of Ashkenazi Jewish (Eastern European) or Russian descent resulting in an affected offspring occurring 1 in 3,600 births.  Carrier frequency is 1 of 300 in the general population.  Canavan Disease is a severe progressive neurologic disorder for which there is no cure.  It is rare outside the Ashkenazi community.  It is estimated that the carrier frequency in the Ashkenazi Jewish population is approximately 1 in 40.  Thus, the risk for an affected offspring in this population approximates 1 in 6,400 births.  Carrier testing is also available for Gaucher Disease.  For all of these diseases, if both parents are identified as carriers the risk that the child will have the disease is 25%.
5. SICKLE CELL DISEASE:  is an inherited disease that destroys red blood cells.  Approximately 8% of African-Americans carry the sickle cell gene.  Doing a simple blood test can screen the parents.  If both parents have the gene, the infant will have a 25% chance of being affected with sickle cell disease.  Tests can then be performed to determine if the fetus is affected.
6. CYSTIC FIBROSIS:   is an inherited genetic disorder that results in excessive mucus production within the lungs and pancreas.  Affected individuals are more prone to pneumonia and diabetes.  Approximately, 1 of 29 Caucasians (Northern and Southern Europeans or Ashkenazi Jewish individuals) carry this abnormal gene.  The Hispanic-Latino frequency is 1 of 46 while it is carried by 1 of 65 with African-American heritage.  Asian-Americans uncommonly carry this gene, approximately 1 out of 90.  A test can screen for the presence of this gene.  Current screening tests identify the most common mutations but not every mutation of the gene can be identified.  Therefore, even patients with a negative test could have a child with CF, although very rarely.  If both patients have the gene, the infant will have a 25% chance of being affected with cystic fibrosis.
7. First Trimester Screening: First Trimester Screening is an optional screening test offered to patients during pregnancy between 11 and 14 weeks. The test combines the results of ultrasound and blood work to estimate your risk of having a baby with Down Syndrome, trisomy 18 or trisomy 13.  An ultrasound examination is done when the fetus is between 11 weeks 1 day and 13 weeks 6 days of age. The ultrasound confirms your baby's age and measures the amount of fluid behind the baby's neck. Immediately after the ultrasound, blood will be obtained from you by finger stick or blood draw. Your blood will be analyzed for 2 substances normally found in all pregnant women. Results are available in about one week.