Prenatal Genetic Screening
by Dr. D’Andrea Bingham
Screening tests are offered to all pregnant women. They can give information on a pregnant woman’s risk of having a baby with certain birth defects or genetic conditions. Some women are at risk for specific genetic disorders based on things such as race,
ancestry, or family history. These women are offered screening for these specific disorders.
However, the routine screenings offered to all pregnant women typically tests for Trisomy 21 (Down Syndrome), Trisomy 18 (causes severe mental retardation, and birth defects ), and open neural tube defects (birth defects involving the baby’s brain or spine).
The following are some of the most common tests available for expectant mothers:
Maternal serum screening or AFP Tetra
This is a maternal blood test to help identify women who are at increased risk for having babies with Trisomy 21, 18, and open neural tube defects. It is performed between 16-21 weeks and measures four proteins in the mother’s blood. These results are combined with other clinical information (such as age and weight) to determine a baby’s risk of having one of the above abnormalities. This test does not tell you for sure that your baby has a problem, but gives a statistical probability of an abnormality. It merely tells you if your baby has an INCREASED likelihood of a problem. For example, results may be reported as “Risk of Down Syndrome is 1 out of 200”. This can then be interpreted as a negative screen or a positive (abnormal) screen. If this test is positive, additional testing (such as a Level 2 ultrasound) will be offered. The AFP Tetra will detect 75-80% of babies with problems, but has a false-positive rate of about 5-6%. This is the most common screening test currently offered.
Level 2 Ultrasound
This is a typical ultrasound but is done by a high-risk obstetrician. It is more in depth than a routine obstetrical ultrasound. There are many reasons your doctor may order a Level 2 ultrasound. They are routinely done in women over age 35, women with a personal or family history of a birth defect, and/or as a follow-up if previous screening test is abnormal. Ultrasounds will not always detect chromosomal abnormalities or birth defects.
First trimester screening
This test combines ultrasound findings with maternal blood work. This can be done between 11- 14 weeks. The ultrasound measures the nuchal translucency (a thickness at the back of the baby’s neck) and combines it with results of blood from a maternal finger stick. This is a very effective screening for Down Syndrome and Trisomy 18, but does not screen for open neural tube defects.
Amniocentesis
This is a more invasive test and is usually performed between 15-20 weeks. Ultrasound is used to guide a needle through the abdomen into the uterus and a small amount of amniotic fluid is withdrawn. Amniocentesis can diagnose most, but not all, chromosomal and genetic diseases, and neural tube defects. Because it is invasive, the complication rate is 1-2%, but it is greather than 99% accurate. This is usually offered in addition to one of the non-invasive screenings rather than as an initial test.
Cell-free fetal DNA
This is brand-new non-invasive prenatal testing. It is a maternal bloodtest for fetal DNA that can be found in the maternal serum. This technology is not currently offered routinely to low risk women or to multiple gestations because of lack of testing in these groups. At this time, it is available to higher risk women, such as those over age 35, women with a history of a child with certain abnormalities, fetuses with abnormal findings on ultrasound, and as a follow-up test for women with abnormal first trimester or AFP Tetra results. This test can be performed as early as the 10th week of pregnancy for Trisomy 21, 18, and 13. It has detection rates of approximately 98% with a very low false positive rate (0.5%). Since it is new, it is a fairly expensive test (3-4 times the cost of the AFP Tetra).
Fortunately, almost all babies in the United States are born healthy and normal. Your doctor can explain all the risks and benefits of available screenings and help you decide which tests are right for you.
