Amniocentesis

How the Test is Performed:
The exact location of the fetus is determined, usually by ultrasound (see pregnancy ultrasound ). The skin of the abdomen is scrubbed and a topical anesthetic may be applied, or a local anesthetic may be injected into the skin. A long, thin needle is inserted through the abdomen and into the uterus. A small amount of fluid is taken from the fluid-filled sac surrounding the fetus (usually about 1 cc. per week of gestation).

How to Prepare:
This test does not require an overnight stay in the hospital. You must sign a consent form. Your bladder must be full for the ultrasound . There are no food or fluid restrictions.

How it Feels:
If an anesthetic is used, there may be a sharp, stinging sensation for a few seconds. When the needle enters into the amniotic sac, a sharp pain lasting a few seconds may be felt. Some women have the sensation of pressure in the lower abdomen when the fluid is withdrawn. After the procedure there may be some minor cramping.

Risks:
There is a slight chance of infection or injury to the fetus. There is even a smaller chance of miscarriage . This test is typically performed when a problem is suspected, so the results outweigh the risk.

Why this Test is Performed:
The test can detect chromosomal disorders, such as Down's syndrome , structural defects such as spina bifida (open spine, where the vertebrae fail to close) and anencephaly (a condition in which the brain is incomplete or missing), as well as many rare inherited metabolic disorders.

Later on in a pregnancy , the test is used to identify suspected problems such as a blood group problem or infection. It is also used to determine whether a fetus in distress is mature enough to survive an early birth.

Late in the pregnancy, lung maturity can be determined using this test.

Normal Values:
The analysis shows no defects in the chromosomes and no excess of alpha fetoprotein (a protein produced by the fetus) or bilirubin .

Abnormal Results:
Amniocentesis can be used to diagnose a large number of genetic and chromosomal abnormalities in the fetus. In addition, it is helpful in the diagnosis of the severity of Rh incompatibility , lung maturity, and neural tube defects (such as spina bifida ).

Some of the diseases for which DNA testing is available include:
  • abetalipoprotenemia
  • acatalasemia
  • Acyl-CoA oxidase deficiency
  • alpha 1-antitrypsin deficiency
  • alpha thallasemia and beta thallasemia
  • Bloom disease
  • Canavan disease
  • Duchenne muscular dystrophy
  • Fabry disease
  • familial dysautonomia
  • Farber disease
  • Fragile X syndrome
  • G6PD (glucose 6 phosphate dehydrogensae deficiency)
  • Gaucher disease
  • GM1 gangliosidosis
  • Hemophilia
  • Hunter syndrome (MPS-2)
  • Huntington chorea
  • Hurler syndrome (MPS-1H)
  • Krabbe disease
  • Lesch-Nyhan syndrome
  • maple syrup urine disease
  • Maroteaux-Lamy syndrome (MPS-6)
  • Metachromatic leukodystrophy
  • Morquio syndrome (MPS-4)
  • Mucolipidosis (ML-1)
  • Mucolipidosis (ML-2)
  • Mucolipidosis (ML-3)
  • Mucolipidosis (ML-4)
  • Multiple sulfatase deficiency
  • Niemann-pick disease
  • phenylketonuria
  • Pompe disease (GSD 2a)
  • GCD4
  • GSD3
  • retinoblastoma
  • Sanfilippo syndrome (MPS-3)
  • Scheie syndrome (MPS-1S)
  • Sickle cell disease
  • Tay-Sachs
  • tyrosenemia type 1
  • Wolman disease
  • X-linked adrenoleukodystrophy



Cost:
The estimated cost is $600 to $900.

Special Considerations:
There may be alternatives to the amniocentesis; this should be discussed with the health care provider.


Amniocentesis is used to determine the health of an unborn baby. Amniotic fluid contains cells that are normally shed from the fetus. Samples of these cells are obtained by withdrawing some amniotic fluid. The chromosome analysis of these cells can be performed to determine abnormalities. In addition, the cells may be cultured and analyzed for enzymes, or for other materials that may indicate genetically transmitted diseases. Other studies can be done directly on the amniotic fluid including measurement of alpha-fetoprotein.