Families who are preparing to welcome a baby should be aware of medical advances that help provide information on the health of both the mother and developing fetus. There are tests available to provide information on various medical issues that can help parents to determine the best course of medical intervention for their situation.
One example of prenatal testing that can provide important medical information involves tests used to determine the presence of Down syndrome. Down syndrome is a genetic condition caused by extra genes of the 21st chromosome. It presents with some degree of cognitive disability and other developmental delays. Some common physical traits include an upward slant of the eyes; flattened bridge of the nose; a single, deep crease on the palm of the hand; and decreased muscle tone. However, not every affected child will have these traits.
The chance of having a child with Down syndrome increases with the mother’s age. At age 35, the odds are about 1 in 350. At age 40, the odds are about 1 in 100. Unlike other genetic syndromes, there is no association between Down syndrome and culture, ethnic group, socioeconomic status or geographic region.
There are three known causes of Down syndrome:
- Trisomy 21, a doubling of one of the genes of the 21st pair of chromosomes, accounts for an estimated 95 percent of children born with Down syndrome.
- Translocation, where an extra part of the 21st chromosome gets stuck onto another chromosome, occurs in 3 to 4 percent of the cases.
- Mosaicism, where the person with Down syndrome has an extra 21st chromosome in some of the cells but not all of them, is responsible for 1 to 2 percent of people with Down syndrome.
Non-invasive testing can screen for markers that are known to show an increased risk for Down syndrome. These can be conducted at two times in the pregnancy. One test can be performed between 10 and 14 weeks of pregnancy by taking a blood sample and performing an ultrasound to measure a nuchal translucency (the amount of fluid at the back of the baby’s neck). The results of the blood test, the nuchal translucency measurement and the mother’s age are then used to estimate the risk for Down syndrome and trisomy 18.
The other test involves a maternal blood test between 15 to 20 weeks of pregnancy. When the results of this blood test are combined with the results from the first trimester blood test and nuchal translucency ultrasound, the detection rate for Down syndrome increases. This test, often called a quin-screen, also provides a personal risk assessment for other possible fetal issues including neural tube defects or an abdominal wall defect.
Based on risk assessment a mother can decide to undergo more definitive diagnostic testing.
Chorionic Villus Sampling (CVS) can be used to detect other genetic diseases like cystic fibrosis, Tay-Sachs disease and sickle cell disease in at-risk fetuses. However, CVS does not detect spinal cord defects. CVS is done at 10 to 12 weeks. CVS involves removing a tiny piece of tissue from the placenta, under ultrasound guidance, either with a needle inserted through the abdomen or a catheter inserted through the cervix. Results are usually received in about two weeks. There is a risk of miscarriage as a result of CVS — 1 in 100 or less, although some centers claim an even lower risk.
Amniocentesis can be used to identify chromosomal problems such as Down syndrome, cystic fibrosis, Tay-Sachs disease and sickle cell disease. An amniocentesis procedure for genetic testing is typically performed between 15 and 20 weeks of pregnancy. Under ultrasound guidance, a needle is inserted through the abdomen to remove a small amount of amniotic fluid.
Amniocentesis detects most chromosomal disorders, such as Down syndrome, with a high degree of accuracy. Testing for other genetic diseases and for neural tube defects, like spina bifida can also be performed at the same time. Again, there is risk of miscarriage as a result of amniocentesis. This risk is very dependent on the facility and doctor performing the procedure and the risk is quoted as high as 1 in 100 to as low as 1 in 800, depending on the facility and operator.
Ultrasound also has a role in both providing screening information for Down Syndrome, but also for other aspects of the pregnancy and non-chromosomal birth defects that can be determined at various points in the pregnancy. Modern 4D ultrasound and a survey of fetal anatomy can provide much more information than just the sex of the fetus.
If you receive positive results on a screening test, start by discussing the results with your doctor and a genetic counselor. Depending on the findings, a medical geneticist may also be helpful. Remember, the purpose of the screening tests and follow up discussions with medical professionals is to give you the greatest amount of information to enable you to make an educated decision for yourself.