Amniocentesis

Amniocentesis is a procedure which involves the withdrawal of a small amount of amniotic fluid for the purpose of genetic testing. Cells in the fluid can be analyzed to determine the baby’s chromosomes and for the detection of open neural tube defects.

The risk of having a baby with certain chromosomal abnormalities increases, as a woman gets older. The most common of these disorders is Down syndrome. There is also an additional risk of having a child with other types of chromosomal abnormalities. Amniocentesis can test for the majority of chromosome problems, such as Down syndrome, associated with a women’s age. Some of these abnormalities are severe in nature, some are mild, and relatively few are unpredictable in their effects on human development. A small part of the sample obtained is also tested for open neural tube defects, such as spina bifida and anencephaly. The risk for neural tube defects is approximately 1 in 1000 and is not related to maternal age.

In order to understand these chromosome problems, it may be helpful to briefly review chromosomes and their function. Chromosomes are the structures in our cells that carry our genetic information or genes. Normally, there are 46 chromosomes in our cells. A child inherits half (23) of their chromosomes from the mother, in the egg, and the other half (23) of their chromosomes from the father, in the sperm. Due to their importance, an error in the number or structure of a chromosome typically leads to serious problems or birth defects. One of the most common chromosome problems is Down syndrome, which occurs when an additional or extra chromosome is present in the cells of the baby. This can be detected by amniocentesis.

Each chromosome carries many genes. Genes have many functions including control of our growth and development, as well as the determination of many traits. Many disorders caused by an error in a single gene can also be detected by amniocentesis for families at risk for these conditions.

Procedure:

Amniocentesis is typically performed after 15 weeks of pregnancy, counting from the first day of the last menstrual period, or as determine by ultrasound examination. During the procedure, a small amount of amniotic fluid is withdrawn from the sac surrounding the baby. An ultrasound is performed before and during the amniocentesis. You will lie down with your abdomen exposed for the procedure. The procedure is performed by inserting a slender needle through the abdomen into the uterus. Amniotic fluid will be removed by applying gentle suction to a syringe attached to the needle. The procedure is typically performed in a minute or two. Including the ultrasound examination prior to the procedure, you can expect to spend approximately 20 to 30 minutes in the procedure room. During your appointment, we will discuss with you the restrictions following the procedure and any follow up recommendations.

Results:

The final results of the chromosome analysis are typically available in approximately 10 to 14 days, although you have the option of obtaining preliminary results in 2 to 3 business days. In order to obtain the final results, the chromosomes within the baby’s cells are studied under a microscope. The number and general shape of the chromosomes are checked. An occasional error has been reported, but the accuracy of amniocentesis for chromosome analysis is greater than 99%. Also, additional testing for specific inherited genetic disorders can be performed on the cells, if necessary.

Rarely, we obtain results which will not allow us to establish with certainty whether or not the fetal chromosomes are normal. These situations are resolved on an individual basis and may require further testing such as examination of the parent’s chromosomes. Very rarely, an insufficient amount of amniotic fluid is obtained when the amniocentesis is attempted. In that event, another amniocentesis may be recommended.

For families at risk for a specific genetic disorder, the timing of the test results will vary depending upon the laboratory used. Your genetic counselor will discuss the turn-around-time for the testing, at the time of your appointment.

Risks:

Although the amniocentesis is a fairly safe procedure, there is some risk involved. Side effects that may occur include the following:

• Cramping
• Bleeding
• Infection
• Leakage of amniotic fluid
• Miscarriage

Unfortunately, all pregnancies have a risk for miscarriage, whether a test was performed or not. In the second trimester (when an amniocentesis is typically performed), the normal risk for miscarriage is between 2-3%. The risk for miscarriage is increased slightly with amniocentesis. In our experience, the risk of amniocentesis-related miscarriage is 1 in 250 to 1 in 300.

There is a very small risk of infection in your uterus. We will be taking precautions against this, but if an infection should occur, it may require treatment.

With prenatal diagnosis, we attempt to rule out certain diseases in the baby. We are not able to state that the baby is healthy in every way. Therefore, if the amniocentesis test results are normal, it gives a high assurance that the baby does not have a chromosomal disorder. However, it does not guarantee a healthy baby. Approximately 3-5% of babies are born with a birth defect or inherited disease, such as deafness, blindness, cleft lip or palate to name a few. Many of these birth defects are undetectable via prenatal diagnosis or ultrasound.

Next Steps:

If you would like additional information regarding our experience with amniocentesis or would like to schedule an appointment for an amniocentesis, please contact our genetic counselors at 773-472-4900.