Women over age 35 are at increased risk of having a child with developmental and chromosomal abnormalities. Physicians and researchers agree, “A woman over age 35 has a 4 to 5 out of 1000 chance of having a child born with Down’s syndrome. While every pregnancy has the potential for defects, the risks for defects increase as women age.

Interestingly, nearly 80% of all children born with Down’s syndrome are born to mothers under age 35.1 In the general population, Down’s syndrome occurs in about 1 in 600 live births and 1 in 150 conceptions2. In the United States, the prevalence is highest amongst Latinos and lowest amongst African Americans3.

Until very recently, the only way to detect chromosomal defects was to do an amniocentesis. While chromosomal abnormalities were detected, the slight increase in miscarriage due to the procedure was unacceptable. So amniocentesis, a definitive diagnostic test for chromosomal abnormalities, has been reserved for women at high risk of having a child with defects.

There are other ways to detect developmental abnormalities. Prenatal screening has evolved rapidly in the past 20 years. And the following tests are now available.

Alpha-Fetoprotein -The alpha-fetoprotein (AFP) test is a screening test done between 15 and 17 weeks gestation to help identify babies who suffer from neural tube defects (Spina Bifida) or Down syndrome. Maternal blood is drawn and the level of AFP is determined. An elevated AFP level indicates that the fetus may have a developmental abnormality.

Triple Screen - A triple screen is a blood test that measures alpha-fetoprotein, human chorionic gonadotropin and unconjugated estriol. The tests are performed to see if the fetus is at higher risk for developmental abnormalities. Triple screens are most accurate when done between the 16th and the 18th weeks of your pregnancy, but they may be done any time between the 15th and 22nd weeks of pregnancy. Expectant parents must remember that these tests are screening tests. The results only show that there may be a problem, they can't prove that there is one. If a developmental abnormality is suspected, a definitive diagnostic test should be performed.

Quadruple Screen - The quadruple screen is essentially the same as the Triple Screen, is but includes a fourth substance in the blood test known as Inhibin-A. The likelihood of identifying pregnancies at risk for Down syndrome is better with the evaluation of Inhibin-A levels. The false positive rate of this test is also lower than the Triple Screen.

The blood tests for Alpha-fetoprotein, the Triple Screen and the Quadruple Screen all test for proteins put out by the developing fetus and placenta that can be measured in the mother’s blood. The AFP test has a high false positive rate and the Triple screen is only accurate in detecting Down’s syndrome about 60% of the time. The Quadruple screen is only accurate in predicting Down’s syndrome about 70 to 75% of the time. Whenever expectant parents receive questionable or elevated results, the next step is to perform definitive diagnostic tests, either an amniocentesis or chorionic villus sampling.

Chorionic Villus Sampling – The Chorionic Villi are tiny fingerlike growths found in the placenta. The genetic material in chorionic villus cells is the same as that in the baby's cells hence they are very reliable detectors of developmental abnormalities. The procedure is generally done between the 10th and 12th weeks of pregnancy and can identify hundreds of family diseases, such as Tay-Sachs disease, hemophilia or Down syndrome. CVS cannot detect neural tube defects such as Spina Bifida.
The chorionic villus sample can be collected by putting a thin flexible tube (catheter) through the vagina and cervix into the placenta and then taking a sample or the sample can be collected through a long thin needle put through the belly into the placenta (as is done with amniocentesis). Ultrasound is used to guide the catheter or needle into the correct spot for collecting the sample. The cells retrieved are examined and if abnormalities exist, they are easily identified.

Amniocentesis - This procedure is used to determine absolute genetic risk for developmental abnormalities. Performed between 15 and 20 weeks of pregnancy, a small amount of amniotic fluid is withdrawn via a long needle inserted through the belly into the amniotic sac. Fetal tissue extracted from the amniotic sac is then examined for genetic abnormalities. There is a slight increased risk of miscarriage with this procedure, so it is only reserved for those women known to be at increased for genetic abnormalities.

In 1995, The First Trimester Risk Assessment Screen became available in the United States. Developed by physicians in London, the test involves performing a special ultrasound to measure the nuchal fold on the back of the neck of the developing fetus. This measurement, combined with the mother’s age and the age of the baby provide a statistical probability of the risk of the baby being born with Down’s syndrome. The First Trimester Risk Assessment screen accurately detects more than 80% of cases of Down’s syndrome and has a low false positive rate. The added benefit of the test is that the ultrasound not only gives information about Down’s syndrome risk, but physicians (and parents) can also take a good look at the baby and look for other potential developmental problems. If there is an increased risk of developmental problems, parents can proceed to definitive diagnostic tests such as amniocentesis or chorionic villus sampling. In the unfortunate case of severe developmental problems, the parents can elect to terminate the pregnancy early sparing the mother potential health complications and the family additional emotional trauma.

Empowerment Points

It is imperative that African American women know about prenatal testing and the differences between screening tests and diagnostic tests. Prenatal screening examinations have come a long way in the last 10-20 years, and now more than ever it is possible to detect developmental abnormalities early and make decisions accordingly. If you are an expectant mother at risk for having a child with developmental abnormalities find out the following:

• What tests are available to you,
• What each test measures,
• What are the potential pitfalls of the test, and
• What information is generated as a result of the test.

Armed with this information, African American mothers can make well-informed decisions about their own health and the health of their babies.

References:

1. Cooley WC, Graham J. M., Down Syndrome: An Update and Review for the Primary Physician, Clinical Pediatrics 1991 30:233.
   
2. Antonfarakis SE, Peterson MB, McInnis MG, et al., The Meiotic Stage Of Nondisjunction In Trisomy 21: Determination By Using DNA Polymorphisms, American Journal of Human Genetics 1992; 5:1411-6.
3. Down Syndrome Prevalence at Birth—United States, 1983-1990. Teratology 1997; 56:31-6.
   

Author: By Darline Turner-Lee, PA-C, ASAM
www.nextstepfitness.com