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- Amniocentesis may be performed for chromosome analysis or the detection of genetic defects and conditions in the fetus.
- Pregnant women over the age of 35 years are candidates for amniocentesis to detect chromosomal abnormalities in the fetus.
- Amniocentesis can also be used to determine the maturity of the fetal lungs or the cause of a fever in the mother.
- For genetic testing, chromosome analysis, and the evaluation of an abnormal alpha feto-protein test, amniocentesis is usually performed between the 16th and 21st weeks of pregnancy.
- The risk of fetal loss from the amniocentesis procedure is less than 1%.
What is amniocentesis?
Amniocentesis is a procedure wherein a sample of fluid is removed from the amniotic sac for analysis. The amniotic sac is found within the uterine cavity. The amniotic sac is the fluid-filled structure inside a pregnant woman's uterus within which the baby lives. Fetal cells, proteins, and fetal urine freely move within this sac.
During amniocentesis, fluid is removed by placing a long needle through the abdominal wall into amniotic sac. Sometimes, the woman's skin is injected first with a local anesthetic, but this is not usually necessary. The amniocentesis needle is typically guided into the sac with the help of ultrasound imaging performed either prior to or during the procedure. Once the needle is in the sac, a syringe is used to withdraw the clear amber-colored amniotic fluid, that resembles urine. The volume of fluid withdrawn depends upon the age of the fetus and the reason for the procedure.
The fluid can then be sent for evaluation of fetal lung maturity, genetic evaluation, evidence of spina bifida (a birth defect in spinal cord development) or other neural-tube defects, the presence of infection, or fetal chromosomal analysis. Chromosomes are structures which contain all of the genetic information within our cells. Amniotic fluid contains numerous free-floating fetal cells that can be grown in the laboratory. When these cells multiply and reach a certain number, their chromosomes are extracted and analyzed. It takes approximately two weeks to perform chromosomal analysis. The fluid also contains proteins, minerals and other compounds that can be tested. These additional studies may require 1 to 7 days to perform. Data obtained from amniotic fluid can help women make informed decisions regarding their pregnancies.
For most patients, amniocentesis is a fairly rapid and comfortable procedure. Some women experience uterine cramping after the procedure. They may also feel faint during or following the test.
Who is a candidate for amniocentesis?
Women over the age of 35 have an increased chance of having a baby with chromosomal abnormalities. Therefore, the current recommendation is that women who will be age 35 or older at the time of delivery be offered a genetic amniocentesis for the testing of fetal chromosomes. Chromosomal abnormalities in the fetus can lead to intellectual disabilities or other birth defects, as seen with Down's syndrome. The possibility of having a baby with a chromosomal defect increases with the age of the mother.
- At age 35, the chance is about 1 in 178.
- By age 40, the rate increases to 1 in 63.
- By age 48, the ratio is 1 in 8.
- For women younger than 35, the risk of complications as a result of amniocentesis outweighs the possibility that the baby has a chromosomal abnormality (see below, "Is Amniocentesis Safe?").
- After 35, the chance of delivering a baby with a chromosomal abnormality may be more common than experiencing the complications associated with this procedure.
If there is a family history of chromosomal problems or history of prior births in which a chromosomal defect was identified, amniocentesis may be recommended. However, the exact benefit of amniocentesis in these situations is uncertain. Many chromosomal defects may not reappear in subsequent pregnancies, and genetic counseling can help a woman decide whether amniocentesis would be of value in these situations.
What does amniocentesis show?
Amniocentesis is very accurate in detecting chromosomal abnormalities and the gender of the fetus. Occasionally, the fetal cells harvested by amniocentesis do not grow in the culture, and no chromosomal data can be obtained. Additionally, physician may be unable to extract fluid from the uterus during amniocentesis due to a variety of technical reasons. In both instances, a subsequent extraction of fluid may be attempted.
Amniocentesis testing for lung maturity can help the doctor make decisions regarding timing of delivery for women in premature labor, those with diabetes, or other medical conditions requiring possible early delivery. Lung maturity testing is not perfect. In rare instances, a baby with lungs that are predicted to perform normally can still experience difficulty breathing.
The AFP (alpha fetoprotein) blood test is currently available and can be used to screen for neural tube defects such as spina bifida, as well as Down's syndrome. An elevated AFP blood level may indicate the possibility that the fetus has a defect in the brain and/or spinal cord development. A low AFP level may indicate the potential for Down's syndrome. When abnormal blood levels of AFP are found, further evaluation with ultrasound and amniocentesis can be done to look for birth defects and chromosomal abnormalities.
Amniocentesis can also help determine if there are specific genetic problems present in the fetus. Testing for Tay-Sachs disease, cystic fibrosis, sickle cell anemia, and other inherited disorders can be performed on samples of amniotic fluid. Checking the cells for specific genes can now be done on the fetal cell cultures. It is also possible to look for a specific protein in the amniotic fluid that can signal a genetic abnormality. Testing for genetic (inherited) diseases is not routinely performed, but is available when necessary.
Another common reason for performing amniocentesis is to determine if the fetal lungs are mature enough so that the baby can be safely delivered before the due date. If the fetal lungs are not mature, an attempt can be made to stop premature labor with medications. If the lung tissues are mature, it may be safe for the premature baby to be born. This test can also help in the timing of delivery when the due date is uncertain.
Occasionally, a pregnant woman may have a fever without an obvious site of infection. In these instances, an amniocentesis may be performed to determine if the cause of the mother's fever is from an intrauterine infection.
The overall possibility of birth defects in any pregnancy is approximately 3%. For defects such as cleft lip, hernia, and extra fingers or toes, amniocentesis will not be diagnostic. Ultrasound may be helpful in detecting these defects in the fetus.
When should amniocentesis be performed?
For genetic testing, chromosome analysis, and evaluation of an abnormal AFP test, amniocentesis is usually performed between the 15th and the 21st weeks of pregnancy. There is a higher rate of miscarriage associated with amniocentesis that is performed earlier than the 15th week. There is no time limit as to how late in the pregnancy amniocentesis can be performed.
If amniocentesis is performed to determine fetal lung maturity, it is often done anywhere between the 32nd and 36th week of pregnancy. In women with diabetes, lung maturity testing may be done as late as the 39th week, because poorly controlled diabetes may delay fetal lung maturity.
Is amniocentesis safe?
Overall, amniocentesis is a safe procedure with the risk of fetal loss generally less than 1%. According to the American College of Obstetricians and Gynecologists Practice Bulletin, there is a procedure-related rate of fetal loss ranging from 1/300 to 1/500 procedures.
Because there is still a slight possibility of pregnancy loss from the procedure, amniocentesis should be performed only when clearly indicated and its benefits have been determined to outweigh any associated risks. Many parents ask for the procedure to "reassure" themselves that everything is perfect. It should be emphasized that an amniocentesis with normal results does not guarantee that the baby will not have a birth or genetic defect. Many birth defects cannot be detected by amniocentesis. Besides fetal loss, other risks associated with amniocentesis include bleeding, cramping and leakage of fluid from the amniotic sac. These problems occur in about 1% of women undergoing the procedure, and they generally resolve on their own. If these symptoms arise following the procedure, the woman is often advised to stop work and remain at bed rest until the symptoms cease. Subsequently activity may be restricted by the doctor. Occasionally, the needle enters the fetus during an amniocentesis. This is generally harmless, but the infant should be monitored for a short time following the testing.
Amniocentesis is a relatively safe and painless procedure that can provide helpful information. The procedure can be offered to selected women after reviewing the anticipated risks and benefits involved.
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ACOG Practice Bulletin No. 88, Dec. 2007. Invasive Prenatal Testing for Aneuploidy. Obstet Gynecol. 2007 Dec;110(6):1459-67.
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