Amniocentesis is one prenatal procedure that may be administered during pregnancy to detect certain abnormalities of the fetus. A sample of fluid is removed from the amniotic sac, which is the bag of fluid that surrounds the fetus. Amniocentesis can detect or rule out certain congenital, genetic or biochemical disorders in the developing child.

During the test, a thin needle is inserted into the abdomen and a small portion of fluid is withdrawn. The fluid is then sent to a laboratory for analysis.

Amniocentesis can reveal whether the fetus has Down syndrome, neural tube defects (e.g., spina bifida), cystic fibrosis (metabolic disorder that causes the lungs, intestines and pancreas to become clogged with thick mucus), blood disorders, infections and other problems.

Amniocentesis is typically a safe procedure for the mother and fetus. However, complications can occur, such as preterm delivery and skeletal deformities. In rare cases, death of the fetus may result.

Although amniocentesis is suggested to women aged 35 years and older and those whose fetuses may be at high risk for certain disorders, it remains an elective procedure. Women should weigh the pros and cons of amniocentesis and discuss their concerns with a physician (obstetrician-gynecologist).

Amniocentesis is a prenatal test in which a sample of fluid is obtained from the amniotic sac (bag of fluid around the fetus) in the mother’s uterus (womb).

Fetal urine makes up most of this fluid later in pregnancy, but it also contains cells shed from the fetus and fetal lung fluids. Laboratory analysis of the cells can yield information about certain congenital and genetic abnormalities in the fetus. The fluid can also provide general information about fetal health.

A physician, usually an obstetrician-gynecologist, performs an amniocentesis by inserting a thin needle into the abdomen and withdrawing a small portion of fluid from the amniotic sac. Ultrasound imaging helps the physician with proper placement of the needle and helps protect the fetus from harm.

Amniocentesis can be performed at different times during a woman’s pregnancy, depending on the desired information:

• Congenital and genetic problems in the developing fetus – usually performed in the second trimester after the 15^th week. Conditions that may be detected include:
  
  
• • Spina bifida. Birth defect in which the spinal column is imperfectly closed, leading to neurological disorders.
    
    
  • Other neural tube defects. These include anencephaly (absence of most of the brain and spinal cord).
    
    
  • Down syndrome. Disorder caused by the presence of an extra 21st chromosome that results in mild to moderate mental retardation, short stature and a flattened facial profile.
    
    
  • Cystic fibrosis. Inherited condition in which the lungs, intestines and pancreas become clogged with mucus.
    
    
• The maturity of the fetus’ lungs. Amniotic fluid can be sent for laboratory analysis to detect the presence of certain chemicals that indicate a high likelihood that the fetus’ lungs are mature if these chemicals are present in specified quantities. Also referred as maturity amniocentesis, this test is typically performed in the third trimester after 32 weeks because fetal lung maturity is unlikely before this time. The fetus’ lungs must be fully developed if the baby is to be delivered safely. Maturity amniocentesis is generally performed when a pregnant woman has signs of preterm labor or early delivery is necessary for medical reasons.
  
  
• Blood disorders in the fetus. These include inherited blood disorders such as sickle cell anemia and hemophilia as well as fetal anemia in fetuses with Rh factor incompatibility. Rh factor is a substance found in red blood cells. Rh incompatibility can occur when a woman who is Rh-negative is exposed to Rh-positive blood cells from her fetus (most people are Rh-positive). This can occur during pregnancy, childbirth or amniocentesis testing. When this happens, the mother’s immune system begins to make antibodies against the Rh-positive blood cells, which can cross the placenta and damage the fetal blood cells. Because of this potential, women who are Rh-negative and are having an amniocentesis are given medication after the test to prevent this potential complication.
  
  
• Infection in the fetus.
  
  
• Uterine infection. Amniocentesis may be used to check for an infection in the uterus after a woman’s membranes have ruptured prematurely.

In some cases, amniocentesis may be performed to remove excess amniotic fluid that has built up in the amniotic sac.

Not all women choose to have an amniocentesis. It is of greatest benefit to expectant parents who are concerned that the developing child may carry a genetic disorder. Parents are most like to consider an amniocentesis if:

• The mother will be age 35 or older by the time the baby is due. Women in this age group are at increased risk of having a baby with a chromosomal abnormality. The most common of these disorders is Down syndrome, a combination of physical and mental disorders resulting from having an extra chromosome. The older the mother, the greater the rate of Down syndrome, according to the March of Dimes:
  
  

  Age of mother	Rate of Down syndrome
  25	One in 1,250 children
  30	One is 1,000 children
  35	One in 400 children
  40	One in 100 children
  45	One in 30 children
  49	One in 10 children

  
  It should be noted, however, that most babies with Down syndrome are actually born to women who are under age 35 because this younger group of women has far more babies.
  
  Research has also shown that the incidence of birth defects rises along with the age of the father at conception.
  
  
• The mother has had an abnormal result from a second-trimester maternal serum marker screening. Increasingly, pregnant women are being screened for the presence of alpha-fetoprotein (AFP) and other substances in their blood. A high level of AFP indicates that the fetus may have a neural tube defect. This is a malformation of the spinal cord or brain, such as spina bifida or anencephaly. A low level of AFP may indicate a chromosome abnormality such as Down syndrome.
  
  
• The parents have previously had a child with a genetic disorder, or either parent is a known carrier of a genetic disorder. Amniocentesis can help diagnose some – but not all – genetic disorders.

Knowing about a genetic disorder in a fetus can help parents prepare to meet the health needs of their developing infant. In some cases, information gleaned from an amniocentesis can lead to treatments to help the child prior to or just after delivery. Some parents may also decide to abort the pregnancy after learning of a grave threat to the developing child’s health.

Amniocentesis checks for many of the same disorders that can be revealed by another form of testing called chorionic villus sampling (CVS). The chief differences between the two tests are:

• Amniocentesis can detect neural tube defects, whereas CVS cannot.
  
  
• CVS is performed earlier than amniocentesis. CVS usually is performed between the 10^th and 12^th weeks of pregnancy.

Prior to the amniocentesis, women should follow all preparatory recommendations provided by their physician (obstetrician-gynecologist). These may include dietary restrictions and medication changes. The test is typically performed in a physician’s office or outpatient clinic.

During the procedure, the woman lies flat on her back on a table. Initially, an ultrasound is performed to obtain an image of the abdomen. During ultrasound, a wand-like instrument is passed over the outside of the woman’s abdomen. This creates sound waves that are translated to a picture that can be viewed on the monitor. The physician will have a clear picture of the fetus, amniotic fluid, placenta and organs in the maternal abdomen.

Once the abdomen has been cleaned with an iodine solution, the physician uses the ultrasound image as a guide while inserting a thin, hollow needle through the skin and into a pocket of amniotic fluid. Great care is taken to ensure that the needle does not harm the fetus or any nearby organs. About 1 ounce (0.3 deciliters) of fluid is withdrawn. In some cases, additional insertions will be necessary to gain the full amount of fluid required. This is especially true if the mother has a multiple pregnancy.

The mother may experience two moments of mild discomfort: when the needle initially penetrates the skin and when the needle penetrates the uterine muscle. The later pain is likely to feel like cramping, and most women can tolerate the discomfort.

Once the needle has been withdrawn, the physician will use ultrasound to check the fetal heartbeat to make sure it is beating normally. In most cases, the procedure takes just a few minutes to complete.

In rare cases, the sample will not yield the information needed to make an accurate diagnosis. However, this is usually not known until the sample has been sent to the laboratory for analysis. As a result, the only alternative may be to repeat the procedure.

Immediately following the amniocentesis, a woman may experience several relatively minor symptoms that should subside quickly. These include:

• Slight cramping
• Small amount of bleeding from the vagina
• Small amount of amniotic fluid leaking from the vagina

If a woman experiences continued symptoms after returning home, she should contact her obstetrician-gynecologist. Such symptoms include:

• Sustained loss of fluid
• Sustained bleeding
• Severe cramping over a period of hours

Women should refrain from strenuous activity and sexual intercourse on the day of the procedure, but typically can return to normal activities the following day.

Once the sample has been obtained, it is sent to a laboratory for analysis. Living cells from the fetus that are contained in the sample are grown in the laboratory for several weeks. They are then tested for chromosomal abnormalities and genetic birth defects. Results are likely to include information about:

• The number of chromosomes in the fetus. This can help indicate the potential presence of a genetic disorder. For example, a fetus with Down syndrome will have an extra chromosome, and a fetus with Turner syndrome will be missing a chromosome.
  
  
• The sex of the fetus.
  
  
• Whether or not the fetus is at increased risk of some birth defects, such as spina bifida.

Results from an amniocentesis will not show whether the baby has an inherited or genetic disease unless the lab is instructed to test for a specific condition. This is normally done only when there is a known family or obstetrical history that places the baby at risk for the same problem.

Test results are usually available within three weeks after the fluid arrives at the lab. More than 95 percent of women with high-risk pregnancies who receive this procedure find out that their fetuses do not have the abnormality for which they have been tested, according to the March of Dimes. However, a normal result from an amniocentesis does not guarantee that the fetus does not have a genetic disorder. Some problems cannot be detected by this procedure.

Though amniocentesis is usually performed safely and successfully, the procedure carries several potential risks. Women and their partners need to weigh these risks against the possible benefits of an amniocentesis. Possible risks include:

• Membrane rupture. Loss of amniotic fluid is more common in women who undergo amniocentesis than in those who do not. This usually stops within a week after the test, but in rare cases, it can continue. If this happens, the mother and fetus will be closely monitored for signs of adverse effects, such as infection or abnormal growth of the fetus. A continuing fluid loss raises the risk of several complications, including:
  
  
• Preterm delivery
• Skeletal deformities
• Lung problems in the fetus
• Death of the fetus
  
  
• Indirect injury to the fetus. Some studies have indicated that amniocentesis may raise the risk of various types of injury to the fetus. Although the link has not been confirmed, it is believed that fetal injuries related to amniocentesis may include clubfoot, hip dislocation and respiratory problems.
  
  
• Infection of the amniotic fluid. This is very rare, but it usually results in loss of the pregnancy. Studies have indicated a possible – though unconfirmed – link between amniocentesis and the transmission of infection from the mother to the fetus. Examples of these infections include:
  
  
• HIV. The virus that causes AIDS.
  
  
• Cytomegalovirus. Any of a group of herpes viruses that attack and enlarge epithelial cells.
  
  
• Hepatitis C. Infection of the liver.
  
  
• Toxoplasmosis. An infection caused by parasites. When contracted by a pregnant woman, it can result in serious damage to the fetus.
  
  
• Miscarriage. In rare cases, an amniocentesis can result in the death of the fetus. This risk may be increased when other circumstances are present, including:
  
  
• Bleeding during the pregnancy
• High levels of maternal serum AFP (alpha-fetoprotein)
• Previous miscarriage
• History of multiple abortions  
  
  
The risk of miscarriage is between one in 400 and one in 200 procedures, according to the Centers for Disease Control and Prevention. Some studies have indicated that the risk of miscarriage is far higher during early amniocentesis (performed during the first trimester) than in second-trimester procedures.

During an amniocentesis, it may be discovered that the woman has Rh negative blood type. Rh factor is a substance found in red blood cells. Rh incompatibility occurs when a woman is Rh-negative and her fetus is Rh-positive (most people are Rh-positive), which can cause health problems for the fetus (e.g., fetal anemia).

As a result, women who have Rh-negative blood type are given a medicine called Rh(D) immune globulin after the test, which helps prevent these problems.

In some cases, information gleaned from an amniocentesis can lead to treatments to help the child prior to or just after delivery. For example, two life-threatening inherited disorders of body chemistry – biotin dependence and MMA (methylmalonic acidemia) – have been detected by amniocentesis and successfully treated in the womb.

Women who undergo amniocentesis may face complex decisions following the procedure. The woman and her partner may choose to terminate the pregnancy if abnormalities are present. This decision can have an emotional and psychological impact on all individuals involved. In addition, if the pregnancy continues, the parents must prepare and adjust to having a potentially handicapped child. Various support groups may help parents with choices related to amniocentesis.