Amniocentesis is a test sometimes done during pregnancy that looks for birth defects and genetic problems in the developing baby. Amniocentesis removes a small amount of fluid from the sac that surrounds the baby in the womb (uterus). It is usually done in a doctor's office or medical center on an outpatient basis. You will probably have an ultrasound first. This helps the perinatologist find out exactly where the baby is in your womb. A local anesthesia is normally not necessary to perform the procedure. In case of increased anxiety, numbing medicine can rubbed onto part of your belly or can be given through a shot in the skin on the belly area. Then the perinatologist inserts a long, thin needle through your belly and into the womb. A small amount of fluid (about 4 teaspoons) is removed from the sac surrounding the baby.

The fluid is sent to a laboratory, where:

  • Genetic studies are done.
  • Levels of alpha-fetoprotein are measured.
  • Other tests may be done.

Results are usually back within 2 weeks.

No specific preparations for the test are needed. Your Rh factor should be clearly known and documented in your maternity papers. You may get a shot of medicine called RhoGAM if you are Rh negative.

Amniocentesis is most often offered to women at increased risk for bearing a child with birth defects. This includes women who:

  • Will be 35 or older when they give birth.
  • Had a screening test result that indicates a possible birth defect or other problem.
  • Have had babies with birth defects in previous pregnancies.
  • Have a family history of genetic disorders

Genetic counseling is an option prior to the procedure. This will allow you to:

  • Learn about other prenatal tests
  • Make an unhurried, informed decision regarding options for prenatal diagnosis. 

This test can also be done to help:

  • Determine how well your baby's lungs are developed late in the pregnancy
  • Detect a blood type mismatch between the mom and the baby  (RH incompatibility))
  • Diagnose an infection in the baby

Amniocentesis can be used to diagnose many different gene and chromosome problems in the baby, including:

  • Down syndrome
  • Rare, metabolic disorders that are passed down through families

It can also help:

  • Determine how well the baby's lungs are developed
  • Detect spina bifida and other neural tube defects
  • Detect Rh incompatibility
  • Diagnose an infection in the baby

A normal result means:

  • No genetic or chromosome problems at the level a DNA lenght exceeding 10 MB were seen in your baby
  • Bilirubin and alpha fetoprotein levels appear normal

Note: Even with normal results after an amniocentesis, your baby may still have other types of birth defects. Therefore, a supplementary anomaly scan at 20 weeks gestation is always recommended.

An abnormal result may mean your baby has: 

  • A gene or chromosome problem, such as Down syndrome and many others
  • Birth defects that involve the spine or brain, such as spina bifida 

Talk to your doctor about the meaning of your specific test results.

  • How the condition or defect may be treated either during or after the pregnancy
  • Special needs your child my have after birth
  • What other options you have regarding maintaining your pregnancy

Risks are minimal, but may include:

  • Infection or injury to the baby
  • Miscarriage
  • Leaking of amniotic fluid
  • Vaginal bleeding

Further information:

– Practical introduction to amniocentesis

Video Amniocentesis (Genesys fertility center)