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.