Who Might See a Prenatal Genetic Counselor?
What Does a Genetic Counselor Do
How are Pregnancies Evaluated for Chromosome Abnormalities?
First Trimester Ultrasound Evaluation
For this evaluation, ultrasound is performed between 11 and 13 weeks 6 days. Evaluation of the developing fetal neck, nasal bone and blood flow measurements can be used to calculate a risk for common chromosome abnormalities. Information regarding the results of the ultrasound are calculated and patients are counseled regarding the results at the time of the appointment.
First Trimester Maternal Serum Screening
Testing for two proteins (bHCG and PAPPA) in the mother's blood can assess the risk for common chromosome abnormalities. This testing may be added to ultrasound information, or done separately if ultrasound information cannot be obtained. Results of this test usually take about a week.
Second Trimester Maternal Serum Screening
This blood test is performed at 16 to 20 weeks of pregnancy. It may be added to the first trimester ultrasound and maternal serum screening, or performed separately if the patient did not have first trimester testing. Patients who had first trimester maternal serum screening often consider this testing if first trimester testing was "borderline", or close to the cutoff for the normal range. It adds additional sensitivity to the first trimester screening tests. It is important that both first and second trimester maternal serum screens be performed at the same laboratory.
Maternal Serum Alpha Fetoprotein Screening
In the second trimester, between 16 and 20 weeks of pregnancy a blood test to measure the amount of Alpha Fetoprotein in the mother's blood may be offered. This test screens for neural tube defects, such as spina bifida. Women who have normal first trimester screening may opt for this test only. It is included with second trimester maternal serum screening, and does not need to be ordered separately.
If family history or screening tests indicate an increased risk for a disorder, amniocentesis may be used to provide diagnosis for the condition. This involves the removal of a small amount of the fluid from around the fetus under the guidance of ultrasound. There is a small risk to the pregnancy, but it is often much less than the risk of the condition of concern. Prior to such testing, genetic counseling is offered to understand the risks, benefits and alternatives to invasive testing.
What About Testing for Women Over 35?
In the past, all women over 35 were offered invasive prenatal testing such as amniocentesis or Chorionic Villus Sampling (CVS) to diagnose chromosome abnormalities such as Down Syndrome. While it is true that the risk for chromosome abnormalities increases with maternal age, most babies with chromosome abnormalities are born to women under 35. This is because women under 35 are having many more babies, and chromosome abnormalities can happen at any age! Therefore age is a poor predictor for who would have a baby with a chromosome abnormality.
Recent advances in prenatal testing can evaluate the risk for common chromosome abnormalities without any risk to the pregnancy. Invasive prenatal testing should be based on the results of these screening tests, parental concern and other personal factors rather than just age alone.