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 many possible birth defects. Most often, it's used to spot common genetic defects (such as Down syndrome) and neural tube defects (such as spina bifida and hydrocephalus). Amniocentesis is usually performed after 15 weeks gestation. This is when the chorionic and amnionic membranes have fused together.
A similar test, chorionic villus sampling (CVS), is sometimes used to detect many of the same genetic abnormalities. However, CVS does not detect neural tube defects. Instead of sloughed fetal cells, CVS examines placental cells, which are genetically identical 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, and may increase the risk of limb deformities.
What Are the Risks?
About one woman in every 200-400 women miscarry as a result of amniocentesis, though recent studies suggest that the rate of miscarriage may be as low as one in 1,000 (0.1%). Amniocentesis done during the first trimester carries a greater risk for miscarriage than amniocentesis done after the 15th week. Less than one woman in every 1,000 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. On the other hand, knowing your baby has a problem may allow you to prepare better for her birth. One study of mothers who continued pregnancies after learning their babies had Down syndrome showed they were much better able to cope than women who discovered at birth that their babies were affected.
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, the doctor carefully inserts a hollow needle through your abdomen, into the amniotic sac, and draws out about four teaspoons of amniotic fluid. 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 the fluid is withdrawn. 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 2 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 syndrome could be to blame. Since alpha-fetoprotein does not need to be cultured in the lab, these results are available right away -- although 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. This includes women who:
- Will be 35 or older when they give birth.
- 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. You and your partner can have certain blood tests 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: A normal amniocentesis shows that your baby’s genetic map is normal. But even with a normal pattern of chromosomes, your baby can have other types of birth defects. About 3 of every hundred babies will have some sort of birth defect.
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 conditions can be treated after birth, and a few can be corrected while the baby is still in the womb, although this type of surgery is still in the experimental stage. Knowing about a condition ahead of time allows parents and physicians to prepare for the child's special needs, even if his condition can't be treated. Discovering your fetus has a genetic problem such as Down syndrome gives you the option to have an abortion.
Q: I have a relative with Down syndrome. Should I have an amniocentesis?
A: Down 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 syndrome. For a 45-year-old, the odds leap to 1/40. Very rarely, Down 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: There are a number of screening tests that can help assess your risk of having a baby with a chromosomal problem. These tests are routinely offered to women of all ages. Such tests include first trimester ultrasound to measure the thickness of the baby’s neck (nuchal translucency), along with maternal serum levels of PAP-A (pregnancy associated protein-A), and maternal serum screenings (including the quadruple screen test, which measures four chemical substances in the mother's blood), and second trimester anatomy ultrasound. These tests are non-invasive, carry no risk of miscarriage or other problems, and can correctly spot 65 - 90% of chromosomal problems. However, none of these tests can confirm that your baby has a problem. To make the diagnosis for sure, doctors need to collect cells from your baby by amniocentesis or CVS.
Zev Williams MD, PhD, FACOG, Reproductive Medicine and Infertility, Weill-Cornell Medical Center, New York, NY. Review provided by VeriMed Healthcare Network.
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