Amniocentesis is a diagnostic test that may be recommended by your health care provider following an abnormal triple test result. Inherited or genetic concerns lead some parents to choose amniocentesis to determine if specific genetic disorders may be present in their baby.

How is amniocentesis performed?

An ultrasound is used as a guide to determine a safe location for the needle to enter the amniotic sac, so the fluid may be safely removed.  A sample of amniotic fluid is collected through the needle. The procedure takes about 45 minutes, although the collection of fluid takes less than five minutes.

The amniotic fluid, which contains cells shed by the fetus, is sent to the laboratory for analysis. Results can take anywhere from a few days to a couple of weeks to be returned.

When is amniocentesis performed?

Amniocentesis is usually performed between 14 and 20 weeks. Some medical facilities may perform amniocentesis as early as 11 weeks.

Amniocentesis can be used later in the third trimester for a few reasons. Your healthcare provider may recommend the procedure if your membranes have ruptured prematurely, in order to assess for uterine infections.  Amniocentesis may also help determine the severity of fetal anemia in babies with Rh disease, and assist your physician to determine whether the fetus requires lifesaving blood transfusions.

Amniocentesis is sometimes done to assess lung maturity. If so, this is done shortly before delivery.

What does the amniocentesis test look for?

Amniocentesis detects chromosome abnormalities, neural tube defects, and genetic disorders. Down syndrome or Trisomy 21 is the most common chromosome abnormality. Genetic disorders include disorders such as cystic fibrosis. The most common neural tube defect is spina bifida.

Amniocentesis is occasionally used late in pregnancy to assess whether the baby’s lungs are mature enough for the baby to breathe on his own.

Amniocentesis also provides access to DNA for paternity testing prior to delivery. DNA is collected from the potential father and is compared to DNA obtained from the baby during amniocentesis. The results are accurate (99%) for determining paternity.

What do amniocentesis results mean?

Amniocentesis is a diagnostic test that detects chromosome abnormalities, neural tube defects and genetic disorders with high levels of accuracy (98-99%).  Although the probabilities of identification are high, this test does not measure the severity of these birth defects.

Alpha-fetoprotein levels and advanced level ultrasounds may assist in assessing the degree of disability that may be present.

What are the risks and side effects to the mother or baby?

Although amniocentesis is considered to be a safe procedure, it is recognized as an invasive diagnostic test that does pose potential risks. According to the Mayo Clinic, it is performed approximately 200,000 times a year. Miscarriage is the primary risk related to amniocentesis.

The risk of miscarriage ranges from 1 in 400 to 1 in 200. In facilities where amniocentesis is performed regularly, the rates are closer to 1 in 400. Miscarriages can occur because of infection in the uterus, water breaking, or labor being induced prematurely.

Although extremely rare, it is possible for the needle to come in contact with the baby. Great precautions are taken by using a sonogram to guide the needle away from the baby. The mother may experience sharp pain when the needle enters the skin and again when it enters the uterus.

Following the completion of the procedure, the mother may experience other side effects that include:

  • Cramping
  • Leakage of fluid
  • Minor irritation around the puncture site

Contact your healthcare provider if these complications continue or get worse.

What are the reasons to test or not test?

The reasons to test or not vary from person to person and couple to couple.
Performing the tests and confirming the diagnosis provides you with certain opportunities:

  • Pursue potential interventions that may exist (i.e. fetal surgery for spina bifida)
  • Begin planning for a child with special needs
  • Start addressing anticipated lifestyle changes
  • Identify support groups and resources

Some individuals or couples may elect not to pursue testing or additional testing for various reasons:

  • They are comfortable with the results no matter what the outcome is
  • Because of personal, moral, or religious reasons, making a decision about carrying the child to term is not an option
  • Some parents choose not to allow any testing that poses any risk of harming the developing baby

It is important to discuss the risks and benefits of testing thoroughly with your healthcare provider. Your healthcare provider will help you evaluate if the benefits from the results could outweigh any risks from the procedure.

Want to Know More?

Compiled using information from the following sources:

1. Williams Obstetrics Twenty-Second Ed. Cunningham, F. Gary, et al, Ch. 13.

2. Mayo Clinic Complete Book of Pregnancy & Baby’s First Year. Johnson, Robert V., M.D., et al, Ch. 6.