Sanford Clinic Women's Health

  • What is an Amniocentesis?

An amniocentesis is a common prenatal test in which a small sample of the amniotic fluid surrounding the baby is removed and studied. Floating freely in the amniotic fluid are some of the baby's cells. A laboratory studies these cells to determine what a baby's chromosomes look like.

  • What can be detected through an amniocentesis?

    Chromosome Abnormalities
    The most common reason an amniocentesis is performed is to provide reassurance and to rule out chromosome abnormalities. The human body is made of millions of cells. Inside each cell are structures called chromosomes which store the genetic “blueprint” for each of us. The body uses these instructions for everything from eye and hair color to growth and development. Normally, each human cell contains 23 chromosome pairs for a total of 46 chromosomes. A missing or extra chromosome can cause problems in how a baby grows and develops, and in many cases leads to birth defects, mental retardation, or both.

    There are various types of chromosome abnormalities. The most common chromosome abnormality that can result in a liveborn child is Down syndrome, which is caused by the presence of an extra chromosome number 21.




 

Inherited genetic disease
Some families are at increased risk for having a baby with an inherited genetic disease. Examples of genetic diseases include muscular dystrophy, cystic fibrosis and sickle cell disease. After an amniocentesis is performed, the cells are sent to a laboratory to perform a specific genetic test for the disease in question.

Neural Tube Defects
The amniotic fluid also contains a protein called alpha-fetoprotein (AFP). If a baby has a neural tube defect (such as spina bifida) the amount of AFP in the amniotic fluid tends to be increased.
  • How accurate are the Amniocentesis results?

An amniocentesis is a highly reliable method of prenatal diagnosis. Approximately 99% of chromosome abnormalities and at least 90% of neural tubes defects can be detected by doing an amniocentesis. If testing for an inherited genetic disease, the accuracy of a test should be discussed with a genetic counselor before genetic testing is pursued, since the accuracy will vary based on the type of disease tested for.

  • Who should consider having an amniocentesis performed?

An amniocentesis should be offered to all women who will be over the age of 35 at the time of delivery. As a woman's age increases, so does her risk for having a baby with a chromosome abnormality. In addition, individuals who have had a child with a chromosome abnormality, individuals who know they carry a chromosome rearrangement and women at risk for having a child with a genetic disease should also be offered an amniocentesis. Finally, women who have had an abnormal 1st trimester screen, 2nd trimester screen, or an abnormal ultrasound should consider this procedure.

  • Are there risks involved with the amniocentesis?

The risk for complications, including miscarriage, is between 1 in 200 and 1 in 300. The risk may be less if the procedure is preformed by a physician known as a perinatologist. The amniocentesis, however, is not routinely offered to all pregnant women because it does carry this small risk for miscarriage.

  • When and how is the amniocentesis performed?

An amniocentesis is normally performed after 15 weeks gestational age. Before the procedure, an ultrasonographer and the perinatologist will perform an ultrasound to look closely at position of the uterus, placenta and the amniotic fluid surrounding the baby. After reviewing the ultrasound, the perinatologist inserts a thin needle through the abdomen and into the uterus to withdraw a few teaspoons of amniotic fluid. The procedure generally only last a few minutes.

  • When will the results be known?

Although it depends on the type of test, results are usually known between 7 and 14 days. The cells need to grow in a laboratory for a few days before any testing can be performed. In some instances, the results could be delayed because the cells grow slowly or the genetic testing takes longer than expected. After the results are known, a patient will be called with the information and a report will be sent to their physician.