Amniocentesis

Amniocentesis is a medical procedure used to detect certain abnormalities before birth.

A small amount of amniotic fluid (the fluid that surrounds and protects the baby in the womb) is withdrawn through a needle inserted into the mother's abdomen. The fluid is evaluated for specific birth defects or other disorders.

Amniocentesis is one of the most common diagnostic tests for pregnant women. When used to detect abnormalities, amniocentesis is typically done between the 15th and 18th week of pregnancy. In some cases, it may be done as early as the 14th week.


Purpose
With amniocentesis, more than 100 genetic (inherited) disorders can be detected early in pregnancy, including Down syndrome and spina bifida. It's also possible to identify the baby's sex, which is helpful when there's a risk of a sex-linked birth defect.

Indications
Your doctor may recommend amniocentesis if:
You're age 35 or older.
You have a biological child with a genetic disorder or neural tube defect (malformation of the spinal cord or brain), or such a defect was detected in a previous pregnancy.
You or your partner have or could be carriers of a genetic disorder.
You have a family history of a genetic disorder.
You have abnormal results from other screening tests, such as an ultrasound or a maternal serum screening, which measures certain substances found in the blood during pregnancy.
You've had several miscarriages.
The baby must be delivered early and his or her lung maturity needs to be checked.
Your doctor suspects you have a uterine infection.
The baby is affected by Rh incompatability, a condition that destroys the red blood cells and causes serious illness.

Patient preparation
Follow your doctor's advice. There's no need to restrict eating or drinking before an amniocentesis. You may be asked to urinate just before the test, however, to make sure your bladder is empty.

Procedure
An amniocentesis takes about 30 minutes. You'll lie down during the procedure, possibly with your abdomen tilted slightly to the left. Your uterus will be monitored for contractions, and your baby's heartbeat will be monitored as well.
To choose the best location to withdraw the amniotic fluid, the doctor will examine the fetus, placenta and umbilical cord with an ultrasound. Next, your abdomen will be cleansed and a local anesthetic may be injected just under the skin. To withdraw a sample of amniotic fluid, the doctor will insert a long, thin needle through your abdomen and into the uterus. You may experience a uterine cramp when the needle is inserted, as well as some cramping or tenderness at the site.

Several genetic and biochemical tests will be done on the fluid, and the fetal cells will be grown and examined for disorders. Through amniocentesis, it's also possible to measure the amount of alpha fetoprotein (AFP) in the amniotic fluid. AFP is a protein made by the liver of the fetus which passes into the amniotic fluid and into the mother's bloodstream. It's useful in the evaluation of possible neural tube defects.


Postprocedure care
For at least several hours after the procedure, you may need to decrease your activity level and avoid heavy lifting or prolonged standing. One to 2 percent of women may experience vaginal spotting or leakage of amniotic fluid. The body quickly replaces the lost fluid within a few days. It's important to report any bleeding or fluid leakage to your doctor immediately. Also contact your doctor if you notice severe abdominal pain or cramping, fever or any change in the baby's normal activity.
If you have Rh-negative blood and your baby may have Rh-positive blood, you'll be given an injection of Rho immune globulin to help prevent Rh disease.


Benefits
An amniocentesis can help the doctor diagnose certain genetic disorders early in a pregnancy. In some cases, treatment can begin even before birth. This information can also help you prepare for the birth of a baby with a birth defect, choose an appropriate hospital for the baby, or decide whether to continue the pregnancy.

Risks
About one of every 200 to 400 women miscarry after amniocentesis. Less often there may be bleeding from the placenta, injury to the baby or infection of the amniotic fluid. Discuss any concerns you may have with your doctor before making a decision about testing.

Considerations
Results are typically available about two weeks after the procedure. Because amniocentesis can't detect all birth defects, including heart problems and cleft lip or palate, a negative test result doesn't offer an absolute guarantee.