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Amniocentesis
By Mayo Clinic staffOriginal Article: http://www.mayoclinic.com/health/amniocentesis/MY00155
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Definition
Amniocentesis is a procedure in which amniotic fluid is removed from the uterus for testing or treatment. Amniotic fluid is the fluid that surrounds and protects a baby during pregnancy. This fluid contains fetal cells and various chemicals produced by the baby.
With genetic amniocentesis, a sample of amniotic fluid is tested for certain abnormalities — such as Down syndrome and spina bifida. With maturity amniocentesis, a sample of amniotic fluid is tested to determine whether the baby's lungs are mature enough for birth. Occasionally, amniocentesis is used to evaluate a baby for infection or other illness. Rarely, amniocentesis is used to decrease the volume of amniotic fluid.
Although amniocentesis can provide valuable information about your baby's health, the decision to pursue invasive diagnostic testing is serious. It's important to understand the risks of amniocentesis — and be prepared for the results.
Why it's done
Amniocentesis is done for different reasons at different stages of pregnancy.
Genetic amniocentesis
Genetic amniocentesis can provide information about your baby's genetic makeup. Generally, genetic amniocentesis is offered when the test results may have a significant impact on the management of the pregnancy — or your desire to continue the pregnancy. Typically, the value of this information can only be assessed by you and your partner.
Genetic amniocentesis is usually done after week 15 of pregnancy, when the two layers of fetal membranes have fused enough that a sample of amniotic fluid can be safely withdrawn. Rarely, genetic amniocentesis may be done as early as week 11 of pregnancy.
You may consider genetic amniocentesis if:
- You had abnormal results from a prenatal screening test. If the results of a screening test — such as the first trimester screen — are positive or worrisome, you may opt for amniocentesis to confirm or rule out a diagnosis.
- You had a chromosomal abnormality or a neural tube defect in a previous pregnancy. If a previous pregnancy was affected by Down syndrome or a neural tube defect — a serious abnormality of the brain or spinal cord — this pregnancy is at higher risk, too.
- You're age 35 or older. Babies born to women age 35 and older have a higher risk of chromosomal abnormalities, such as Down syndrome.
- You have a family history of a specific genetic disorder, or you or your partner is a known carrier of a genetic disorder. In addition to identifying Down syndrome and spina bifida, amniocentesis can be used to diagnose many other conditions — such as cystic fibrosis. This requires specialized testing of the amniotic fluid, however, so there must be a reason to test for these conditions.
Maturity amniocentesis
Maturity amniocentesis can determine whether a baby's lungs are ready for birth. This type of amniocentesis is done only if early delivery — either through induction or C-section — is being considered to prevent pregnancy complications for the mother. It's usually done between 32 and 39 weeks of pregnancy. Earlier than 32 weeks, a baby's lungs are unlikely to be fully developed.
Other reasons for amniocentesis
Occasionally amniocentesis is used to:
- Evaluate a baby for infection or other illness
- Decrease the volume of amniotic fluid
- Diagnose a uterine infection
- Evaluate the severity of anemia in babies who have Rh disease — an uncommon condition in which a mother's immune system produces antibodies against a specific protein on the surface of the baby's blood cells
Risks
Amniocentesis carries various risks, including:
- Miscarriage. Second-trimester amniocentesis carries a slight risk of miscarriage — between 1 in 300 and 1 in 500. Research suggests that the risk of miscarriage is higher for amniocentesis done before 15 weeks of pregnancy.
- Cramping and vaginal bleeding. Cramping is possible after amniocentesis. Some women experience a small amount of vaginal bleeding.
- Needle injury. During amniocentesis the baby may move an arm or leg into the path of the needle. Serious needle injuries are rare.
- Leaking amniotic fluid. Rarely, amniotic fluid leaks through the vagina after amniocentesis. If the leak seals, the pregnancy may proceed normally. Sometimes, however, the leakage leads to orthopedic problems for the baby.
- Rh sensitization. Rarely, amniocentesis may cause the baby's blood cells to enter the mother's bloodstream. If you have Rh negative blood, you'll be given a drug called Rh immunoglobulin after amniocentesis to prevent you from producing antibodies against your baby's blood cells.
- Infection. Rarely, amniocentesis may trigger a uterine infection.
- Infection transmission. If you have an infection — such as hepatitis C, toxoplasmosis or human immunodeficiency virus — amniocentesis may cause transfer of the infection to your baby.
Remember, genetic amniocentesis is typically offered when the test results may have a significant impact on management of the pregnancy. Ultimately, the decision to have genetic amniocentesis is up to you. Your health care provider or genetic counselor can help you weigh all the factors in the decision.
Maturity amniocentesis is often suggested when early delivery would be best for the mother. With minimal risks, maturity amniocentesis can offer assurance that the baby is ready for birth.
How you prepare
Before amniocentesis, you can eat and drink as usual. Your bladder must be full before the procedure, however, so drink plenty of fluids before your appointment. Your health care provider may ask you to sign a consent form before the procedure begins. You may want to ask someone to accompany you to the appointment for emotional support or to drive you home afterward.
What you can expect
Amniocentesis is usually done in an outpatient facility or the health care provider's office.
During the procedure
First, your health care provider will use ultrasound to determine the baby's exact location in your uterus. You'll lie on your back on an exam table and expose your abdomen. Your health care provider will apply a special gel to your abdomen and then use a small device known as an ultrasound transducer to show your baby's position on a monitor.
Next, your health care provider will clean your abdomen with an antiseptic. Generally, anesthetic isn't used. Most women report only mild discomfort during the procedure.
Guided by ultrasound, your health care provider will insert a thin, hollow needle through your abdominal wall and into the uterus. A small amount of amniotic fluid will be withdrawn into a syringe, and the needle will be removed. The specific amount of amniotic fluid withdrawn depends on the number of weeks the pregnancy has progressed.
You'll need to lie still while the needle is inserted and the amniotic fluid is withdrawn. You may notice a stinging sensation when the needle enters your skin, and you may feel cramping when the needle enters your uterus. The entire procedure usually takes about an hour, although most of that time is devoted to the ultrasound exam. In most cases, the fluid sample is obtained in less than two minutes. The small amount of amniotic fluid that's removed will be replaced naturally.
After the procedure
After the amniocentesis, your health care provider may use ultrasound to monitor your baby's heart rate. You may experience cramping or a small amount of vaginal bleeding immediately after the amniocentesis. Your health care provider may suggest resting after the procedure. You may want to ask someone to drive you home. You'll likely be able to resume normal activities the next day.
Meanwhile, the sample of amniotic fluid will be analyzed in a lab. For genetic amniocentesis, some results may be available within a few days. Other results may take one to two weeks. Results of maturity amniocentesis are often available within hours.
If you develop a fever after amniocentesis or if vaginal bleeding, loss of vaginal fluid or uterine cramping lasts more than few hours, contact your health care provider.
Results
Your health care provider or a genetic counselor will help you understand your amniocentesis results.
For genetic amniocentesis, test results can reliably rule out or diagnose various genetic disorders, such as Down syndrome. Follow-up testing isn't usually necessary. Amniocentesis can't identify all birth defects, however.
If amniocentesis indicates that your baby has a chromosomal problem or a hereditary disorder that can't be treated, you may be faced with wrenching decisions — such as whether to continue the pregnancy. Seek support from your health care team and your loved ones during this difficult time.
For maturity amniocentesis, test results can reliably indicate a baby's lung maturity. If you need to deliver the baby early, this information offers reassurance that your baby is ready for birth.
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- Cystic fibrosis: Prenatal screening and diagnosis. American Congress of Obstetricians and Gynecologists. http://www.acog.org/publications/patient_education/bp171.cfm. Accessed Feb. 9, 2010.
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