Amniocentesis Genetic Testing
Amniocentesis at a glance
- Amniocentesis involves taking a sample of amniotic fluid through a thin, hollow needle that is inserted through the mother’s abdomen and into the amniotic sac surrounding the baby.
- The amniotic fluid contains cells shed from the baby’s skin, lungs, and urinary tract. These cells and fluid may be checked for certain genetic problems.
Why have amniocentesis testing?
Chromosomes are tiny packets of genetic information that decide a baby’s growth and development. In most normal cells, there are 46 chromosomes.
With amniocentesis, the baby’s cells from the amniotic fluid are cultured (grown) in a laboratory, and the chromosomes of about 15 cells are studied through a microscope.
Sometimes there is either additional or missing chromosomal material that indicates the baby may be born with abnormalities. Chromosome analysis will be able to identify Down syndrome (in which there are 47 chromosomes due to an extra number 21 chromosome) and other types of chromosomal problems in the developing baby.
A chemical known as Alpha-Fetoprotein (AFP) is a protein that is normally present in the amniotic fluid in small amounts. If there is an opening along the developing baby’s spinal cord, brain, or abdominal wall, the AFP may leak out in large amounts.
This leads to unusually high levels of AFP in the fluid surrounding the baby, which can be measured during amniocentesis testing.
In addition to the chromosome analysis and AFP test, there are several hundred other tests that can be performed on the amniotic fluid. Because of the limited amount of fluid and the high cost of these additional tests, they are not done routinely on all amniotic fluid samples.
Genetic counseling is helpful in identifying whether the pregnant mother carries risk for other genetic problems that could develop in the baby and be detected by amniocentesis.
It is important to realize that no test can detect all birth defects. There is a 2 percent to 3 percent chance that any pregnancy may produce a child with a birth defect or serious health problem. Many of these problems are not detectable by amniocentesis and ultrasound.
Thus, normal test results do not necessarily guarantee a perfectly healthy baby. However, women who have an increased risk for having a baby with a problem that could be detected through amniocentesis are encouraged to do so.
An ultrasound exam will be performed before the amniocentesis. An ultrasonographer will apply warm gel and move an ultrasound transducer across the abdomen. Sound waves bounce off the tissues of the fluid-filled spaces in the uterus, and a two-dimensional, black and white picture will display on a monitor.
An ultrasound is performed before the amniocentesis testing for the following reasons:
- To make sure the woman is pregnant and there is only one baby. If there is more than one baby, fluid can be extracted from both amniotic sacs.
- To measure the developing baby and to determine how far along the woman is in the pregnancy. Amniocentesis is most commonly performed after 16 weeks from the mother’s last menstrual period. This is the best time to have a chance of getting enough fluid, while also allowing time to discuss options and make decisions after the results are available.
- To assess the anatomy of the baby and ensure healthy development of key features including the face, brain, spine, stomach, bladder, kidneys, and heart.