Amniocentesis | Obstetrics | Dartmouth-Hitchcock
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Amniocentesis

You should consider amniocentesis if you want to find out whether your baby has one of a particular group of genetic conditions.

About amniocentesis

Amniocentesis is a good option in some pregnancies but not in every pregnancy, and the decision to test is yours alone. It is typically performed between 16 and 20 weeks, but it can be performed any time after 15 weeks of pregnancy.

Any woman who is pregnant, regardless of her age, is at risk to have a child with a chromosome condition, and therefore can choose to have an amniocentesis. Factors that put a woman at higher risk include her age, an abnormal screening test or ultrasound finding, or a family history of certain chromosome conditions, inherited genetic diseases, or open neural tube defects.

In the past, it was routine in this country to offer amniocentesis to women who were age 35 and older at delivery. This practice is now changing. Amniocentesis and prenatal screening tests are now available to all pregnant women.

Conditions the test can find

Amniocentesis looks at the baby's chromosomes by extracting skin cells from the fluid surrounding the baby. The laboratory will then grow and study these cells. Chromosomes are small packages of DNA that contain all of our genes, which provide the instructions for human development. In healthy individuals, each cell contains a total of 46 chromosomes. Amniocentesis can detect if the baby has an extra or missing chromosome or large pieces of chromosome material, which could mean there is a problem with the development of the baby that may lead to serious birth defects and/or mental retardation.

  • Amniotic fluid tests for open neural tube defects (NTDs), like spina bifida, by measuring a protein in the fluid called alpha fetoprotein. NTDs are a type of birth defect involving the spinal cord and/or brain that occur in one in 1000 babies. They can cause an increase in the level of AFP in the amniotic fluid. Most of these are seen on an ultrasound done at the time of your amniocentesis.
  • If indicated, amniocentesis can also test for certain inherited genetic diseases. If a couple is at known risk to have a child with a certain genetic condition such as cystic fibrosis, sickle cell disease, or Tay-Sachs disease, it is usually possible to test the baby's DNA by amniocentesis to see if the baby has the condition.
  • Down syndrome, or trisomy 21, is the most common chromosome condition detected. It is caused when there is an extra 21st chromosome in each cell, creating a total of 47 chromosomes. People with Down syndrome have mild to moderate mental retardation and may have certain birth defects.
  • Trisomy 18, or Edward syndrome, is less common than Down syndrome. It is caused when there is an extra 18th chromosome. Few babies with trisomy 18 survive to birth, and most of those who do die within the first year of life. Trisomy 18 causes mental retardation, heart defects, very poor growth and other problems.

The procedure

An amniocentesis lasts only a few minutes.

  • The doctor uses an ultrasound to figure out the best area on the mother's stomach to insert the needle, while avoiding the mother's belly button, the baby, and the placenta when possible.
  • The doctor will then use a thin needle to withdraw one to two tablespoons of amniotic fluid. Usually the doctor will need only one attempt to obtain this fluid, except in the case of fraternal twins, where one insertion is needed for each baby. In rare cases, a second insertion will be needed when there is a single baby to obtain enough fluid.
  • Very rarely, fluid cannot be obtained and the test cannot be performed.

Afterwards

  • We recommend limiting physical activity for 24 hours after the procedure. This means no heavy lifting, excessive exercise or sexual activity.
  • Women with an Rh-negative blood type would need a Rhogam shot within 72 hours after undergoing amniocentesis to avoid certain problems.

Processing the test

  • On average, results take about 10-14 days. Sometimes the results will take longer than expected if the fetal cells do not grow in the laboratory as quickly as expected.
  • Very rarely, cells will not grow at all. If the cells do not grow, then the results of the amniocentesis, or any special tests that were ordered with the amniocentesis, will not be reported. In this case, a second amniocentesis may be needed.
  • In situations where information is needed more quickly from the amniocentesis, preliminary results with F.I.S.H. (fluorescent in-situ hybridization) may be available in two to three days.
  • If any special genetic testing was done with the amniocentesis, the timing for these results will depend on the condition being tested for, and may take anywhere from one to six weeks.

Accuracy of amniocentesis

  • For Down syndrome and trisomy 18, this test is more than 99 percent accurate.
  • The accuracy for open neural tube defects is approximately 98 percent.
  • If any special genetic testing was done on the amniocentesis sample, the accuracy will depend on the condition being tested and on the laboratory doing the testing.

Safety and side effects

There is a risk of miscarriage when a woman has an amniocentesis. The risk of miscarriage is about one in 300, or about one third of one percent. Often we cannot determine the exact cause of a miscarriage, and it is possible that an underlying fetal problem was present. However, sometimes we can identify the cause, such as an infection or leakage of amniotic fluid because the membranes surrounding the baby did not seal correctly.

Since amniocentesis requires a needle to pass through the mother's stomach into the uterine wall (which is a muscle), some women may experience some cramping or a pressure sensation. Mild cramping for several hours after the procedure is common. Soreness, tenderness, or bruising at the site of the needle puncture may also occur. In rare cases, women will have leakage of some amniotic fluid or bleeding from the vagina.

And remember...

All pregnancies have about a three- to five-percent chance of resulting in a child with some type of condition that our studies would not detect. A normal amniocentesis result does not mean that your baby is healthy in every way. Babies can be still be born with a birth defect, inherited disease or mental retardation, even with normal chromosomes.

If you have any further questions about amniocentesis, or if you are having trouble deciding if you want this test, speak with your pregnancy care provider or a genetic counselor.

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