What is amniocentesis and why is it performed?

Amniocentesis is a medical procedure used to remove a sample of amniotic fluid, the fluid that surrounds a developing fetus. Physicians run tests on fetal cells in the fluid (genetic amniocentesis) to diagnose or rule out inherited genetic diseases or chromosome abnormalities, such as Down syndrome. Doctors use another type of test – maturity amniocentesis – to determine if a baby’s lungs are developed enough for it to survive if it must be delivered prematurely.

Healthcare providers typically recommend amniocentesis between the 14th and 20th week of pregnancy. If an obstetrician is considering inducing early delivery to manage risk to the mother’s or baby’s health, he or she may recommend the test after the 32nd week to gauge the maturity of the fetus’s lungs.

Who is offered amniocentesis?

Healthcare providers do not offer this test to everyone. Amniocentesis is most often recommended for pregnant women who:

• Are over the age of 35 and more at risk of having a child with Down syndrome or other genetic conditions
• Received abnormal results from a first trimester screening test, such as a triple test
• Have had a child – or baby that was not carried to term – with a genetic birth defect
• Have – or whose husbands have – a family history of certain diseases or birth defects, such as cystic fibrosis or sickle cell disease

How is amniocentesis performed?

Amniocentesis is usually performed in a doctor’s office, an outpatient facility or a hospital clinic. A medical professional uses an ultrasound to locate the fetus, then guides a long, thin needle through the mother’s abdomen into her uterus to extract a few teaspoons of fluid from the amniotic sac. The baby’s heartbeat and the mother’s blood pressure and heart rate are monitored throughout the procedure, which typically takes between 30 minutes and one hour.

What might the results show?

Genetic amniocentesis may give parents information about:

Chromosomal abnormalities – Abnormalities such as Down syndrome (Trisomy 21) and rarer Patau syndrome (Trisomy 13) or Edwards syndrome (Trisomy 18) occur when a fetus has an extra chromosome, which can be identified through this test.

Neural tube defects – A baby’s neural tube eventually develops into its brain, spinal cord, spinal nerves and backbone. Amniocentesis can definitively detect neural tube defects, although doctors can use advanced ultrasound as a noninvasive alternative.

Genetic disorders – Many hereditary disorders can be detected using the cells from amniotic fluid. These include problems with the body’s chemistry, such as cystic fibrosis, Tay-Sachs and sickle cell disease, or hereditary disorders, such as some types of muscular dystrophy, hemophilia or Huntington’s disease. These diseases are usually tested for only if the family has a history of the condition.

Maturity amniocentesis determines the maturity of a baby’s lungs, which is important to know if a baby needs to be delivered early.

Doctors less commonly use amniocentesis to test for Rh incompatability or intrauterine infections.

What side effects and risks are involved with the test?

Some women say the procedure does not hurt at all. Others report cramping when the needle enters the uterus or slight pressure as the fluid is drawn. After amniocentesis, a woman may have mild cramping or light vaginal bleeding. If the cramping or bleeding lasts longer than a few hours, if the bleeding is heavy, or if a fever is present, the patient should contact her doctor. Women who are Rh incompatible with the fetus require an Rh immunoglobulin shot to prevent complications.

Although amniocentesis is a routine procedure and is usually completed without complications, there are some risks associated with the procedure, including a small risk of infection, bleeding that is difficult to stop, leaking of amniotic fluid and/or miscarriage. Studies indicate that miscarriage rates range from one in every 1,000 procedures to as many as one in every 200 procedures. Factors include the week in the pregnancy the procedure is performed and the experience of the doctor performing it.

What is the outcome of amniocentesis?

The results usually show that the baby is free from certain hereditary problems; approximately 95% of women who undergo amniocentesis have normal outcomes. The results are very accurate in identifying certain genetic disorders and are usually available within two weeks. However, amniocentesis cannot identify all birth defects, such as heart conditions, club foot or cleft lip and palate.