Are you confused about various prenatal tests? Get the scoop on genetic tests and what they mean for you and your baby-to-be.
Doctors use several tests to help examine your unborn child’s genetic makeup. Some of these tests examine the fetus’ chromosomes, others use biochemical or DNA tests. These accurate tests can identify many (but not all) genetic birth defects.
How Genetic Testing Works
New techniques in genetic testing allow doctors to acquire reliable specimens with little risk of injuring the fetus or mother. Tissue cells are obtained through a variety of procedures (the most common are listed within this article). The cells are then grown in the lab and stimulated to divide with chemicals. The chromosomes of dividing cells are isolated, stained, and examined.
What It Is: Babies normally float in a sea of amniotic fluid within the uterus. This fluid contains living fetal cells that have been released from the baby’s skin and other tissues. Withdrawing fetal cell-containing amniotic fluid provides genetic material for examining your baby’s chromosomes.
Other genetic and biochemical testing of the fluid is also possible. For example, measurement of acetylcholinesterase and amniotic fluid alpha-fetoprotein (AFP) can also be performed on amniotic fluid. These two tests assist in prenatally diagnosing spina bifida and other birth defects.
When It Is Performed: This procedure is usually done at 15 to 17 weeks gestation; results generally return in seven to 14 days.
The Risks: Amniocentesis is considered much safer today than previously thought. The advent of ultrasound use during this procedure is part of the credit, as it is now much easier for doctors to track where the amniocentesis needle is within a pregnant woman. However, there are still risks. According to the Centers for Disease Control and Prevention (CDC), the risk of losing your baby from amniocentesis is between one in 200 and one in 400, depending on the skill and experience of the doctor performing it.
You can lessens this risk by seeking out a doctor who’s done the procedure many times (look for someone who performs amniocentesis tests at least 50 times per year) and also a doctor who uses ultrasound throughout the procedure.
Chorionic Villus Sampling (CVS)
What It Is: Another way of obtaining fetal tissue is to take a small amount of the placental tissue from within the uterus, which has the same genetic composition as the fetus. To perform a CVS test, a needle is inserted into the placental attachment to the uterus and a small amount of tissue is removed and sent for analysis. The placenta can be approached either through the cervix or through the mother’s abdominal wall.
When It Is Performed: This procedure can be done at 10 to 12 weeks, which is much earlier than routine amniocentesis. The results are available in about five to 10 days, also much quicker than with routine amniocentesis. The specimens obtained from this procedure can be contaminated with maternal tissue about one percent of the time (compared to about 0.2 percent in amniocentesis); this can confuse interpretation of the genetic tests. There are also rare circumstances when the abnormalities seen in the placenta may not be present in the fetus. Not only do you need an experienced professional performing the test, you also need one who can accurately interpret the test results.
The Risks: Having CVS increases the risk of fetal loss by about 1.2 percent over your risk without the procedure. Your actual risk is dependent on the skill of the person doing the procedure and your specific condition (again, look for a physician who performs more than 50 of these tests annually).
Percutaneous Umbilical Cord Blood Sampling (PUBS)
What It Is: The PUBS or cordocentesis procedure takes fetal blood directly from the umbilical cord to obtain the fetal cells or plasma needed for DNA, chromosomal, or biochemical testing. This test is used only in certain circumstances and is usually only when amniocentesis or chorionic villus sampling would not be adequate. It provides the most accurate information and the most rapid results.
During PUBS, a needle is inserted through the pregnant woman’s abdominal wall, into the amniotic cavity, and into the fetal umbilical cord. Test results are usually ready much faster than with amniocentesis, often within 48 to 72 hours.
When It Is Performed: PUBS can be done after week 17 of pregnancy.
The Risks: The PUBS procedure increases your risk of fetal loss by about two percent. The procedure requires considerable skill and should be done only at referral centers.
Technology is now available to identify many genetic disorders in your fetus before delivery. Most of these tests examine only for a single disorder or group of disorders; your doctor must know which disorder he or she suspects to obtain the appropriate test.
The most important consideration is how the information will be used. For some couples, genetic testing helps your doctor manage your pregnancy and delivery. It may also decrease your anxiety.
You should know why your doctor recommends the testing and how it may influence your pregnancy, delivery, and newborn baby. Genetic counseling is available to help you in this process.