Amniocentesis - series
Amniocentesis is a procedure used to draw a sample of the amniotic fluid that surrounds and cushions the baby in the womb. This fluid contains cells that have been sloughed off of the developing fetus
Amniocentesis is most often used to detect genetic abnormalities, though the results can also reveal the baby's sex, gauge lung maturity (if done close to term), detect amniotic fluid infections, or spot other pregnancy complications.
What Is It?
Amniocentesis can diagnose or rule out hundreds of possible birth defects. Most often, it's used to spot common genetic defects (such as Down's syndrome) and neural tube defects (such as spina bifida and hydrocephalus). Amniocentesis is usually performed at 15 to 18 weeks gestation, though it can be done as early as 11 or 12 weeks.
(A similar test, chorionic villus sampling (CVS), is sometimes used to detect many of the same abnormalities. Instead of sloughed fetal cells, CVS examines placental cells, which are genetically similar to the fetus. Since CVS can be done earlier in the pregnancy, and since results are available within 48 hours, it may be preferable for women who need quick answers about their baby's health. CVS carries a higher risk of miscarriage than amniocentesis, though, and may increase the risk of limb deformities.)
What Are the Risks?
Between one in 400 (0.25%) and one in 200 (0.50%) women miscarry as a result of amniocentesis. Amniocentesis done during the first trimester carries a greater risk for miscarriage than amniocentesis done after the 15th week (fig. 1). Less than one in 1,000 (0.001%) women develop a uterine infection after amniocentesis.
How Can I Prepare?
Amniocentesis is used strictly to diagnose problems during pregnancy -- it does not treat these problems. So before you have the test done, think about how you might use the information it provides. Some birth defects are incompatible with life, others are easily correctable, and still others fall somewhere in between. Would you terminate your pregnancy if your baby had a severe defect? Will you know the results (which may take up to two weeks) in time to consider termination? Weigh the benefits and risks. If you wouldn't terminate your pregnancy under any condition, for instance, the test may cause more worry than it's worth.
If you decide to proceed with the amniocentesis, your medical team will give you detailed instructions on how to prepare for the actual procedure.
What Will Happen?
First, an ultrasound is done to pinpoint the exact position of the fetus and placenta. This allows your doctor to determine the safest place to insert the needle. Guided by ultrasound, he carefully inserts a hollow needle through your abdomen, into the amniotic sac (fig. 2), and draws out about four teaspoons of amniotic fluid (fig. 3). (Don't worry, your baby quickly replaces any fluid that is lost.) When the procedure is done, a quick ultrasound is done to make sure the fetal heartbeat is still normal
If a local anesthetic is used, you may feel a stinging sensation for a few seconds. You may also feel slight pain or cramping when the doctor inserts the needle and pressure in your lower abdomen when he withdraws the fluid. Some women report no pain or discomfort, but most physicians recommend several hours of rest after the procedure.
After they are removed, the fetal cells are grown in a laboratory and then analyzed. Your doctor will get the complete results within two weeks. In about 95% of cases, no abnormalities are found.
Lab technicians will also measure the level of alpha-fetoprotein in the amniotic fluid. If it's high, the baby could have a neural tube defect. If it's low, Down's syndrome could be to blame. Since alpha-fetoprotein does not need to be cultured in the lab, these results are available right away - though a potential problem must be confirmed by subsequent genetic analysis.
Who Should Have Amniocentesis?
Amniocentesis is typically offered only to women at increased risk for bearing a child with birth defects (fig. 4). This includes women who:
- Will be 35 or older when they give birth. (Down's syndrome and other genetic abnormalities are more common among older mothers.)
- Have a screening test or exam result that indicates a possible birth defect or other problem.
- Have had birth defects in previous pregnancies.
- Have a family history of genetic disorders. (If birth defects run in your family, seek genetic counseling before you have amniocentesis - and if possible, before you get pregnant. Certain blood tests can be performed on you and your partner to determine your chances of having a baby with a birth defect.)
Frequently Asked Questions
Q: Do normal amniocentesis results mean I will have a healthy baby?
A: Most often, amniocentesis shows no obvious disorders, and the test itself is more than 90% accurate. Even so, no prenatal test can guarantee a healthy baby. About two of every 100 babies with normal amniocentesis results do, in fact, have birth defects.
Q: Can doctors treat any defects diagnosed by amniocentesis?
A: Scientists have been working on possible cures and treatments for a variety of fetal conditions. Many can be treated after birth, and a few can be corrected while the baby is still in the womb - though this type of surgery is still in the experimental stage. Even when a condition can't be treated, knowing about it ahead of time allows parents and physicians to make decisions and prepare for the birth of a child with special needs.
Q: I have a relative with Down's syndrome. Should I have an amniocentesis?
A: Down's syndrome occurs when the cells of a fertilized egg divide abnormally, creating an extra 21st chromosome. Maternal age - not family history - seems to be the most important risk factor. A 20-year-old mother, for example, has a 1/2000 chance of delivering a baby with Down's syndrome. For a 45-year-old, the odds leap to 1/40. Very rarely, Down's syndrome is inherited when a father or mother carries an egg or sperm with a defective 21st chromosome. If you think you have an increased risk, discuss the pros and cons of genetic testing with your doctor or a genetic counselor.
Q: Are there alternatives to amniocentesis?
A: Yes, and they are routinely offered to women of all ages. Such tests include ultrasound and maternal serum screenings (including the quadruple screen test, which measures the level of alpha-fetoprotein and three other substances in the mother's blood). These tests are non-invasive, carry no risk of miscarriage or other problems, and can correctly spot 65% to 85% of birth defects.
Review Date: June 29, 2001
Reviewed By: Peter Chen, M.D., Obstetrics and Gynecology, University of
Pennsylvania School of Medicine. Review provided by VeriMed Healthcare
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