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What it is

With amniocentesis, your doctor uses a thin needle inserted into your abdomen to take a small sample of amniotic fluid from the sac surrounding your baby. The two common types are genetic amniocentesis and maturity amniocentesis:

• Genetic amniocentesis. It can give you and your doctor information about your baby's genetic makeup before your baby is born.

• Maturity amniocentesis. With this test, the fluid is analyzed to find out if the baby's lungs are mature enough to function normally at birth.

The amniotic fluid is a clear liquid that envelops your baby in the uterus and provides a cushion against everyday bumps and jars. The fluid consists mostly of urine from your baby. It also contains cells that your baby has shed.

With genetic amniocentesis, a sample of these cells can be collected and grown in a laboratory. From that sample, the chromosomes and genes can be checked for abnormalities, such as Down syndrome. The amniotic fluid sample can also be tested for signs of neural tube defects, such as spina bifida.

When it's administered

Genetic amniocentesis is usually performed between the 15th and 19th weeks of gestation. At this point, your uterus generally contains enough amniotic fluid, and your baby is still small. It can be done earlier, but that may increase the risk of a pregnancy loss.

Maturity amniocentesis is done when there may be a reason to deliver the baby before the due date. It indicates whether the baby's lungs are ready for birth. It's usually done from 32 to 39 weeks of gestation.

How it's done

Amniocentesis can be done in your doctor's office. It doesn't require a hospital stay. Before the procedure begins, your doctor or genetic counselor will likely discuss the test with you and help you consider what a positive or negative result might mean. An ultrasound is performed to show the exact location of your baby in the uterus. Together, the discussion and preliminary ultrasound take about 45 minutes to an hour.

During amniocentesis, an ultrasound transducer shows on a screen the positions of your fetus and the needle, enabling your doctor to safely withdraw a sample of amniotic fluid for testing.

After the baby's position is determined, the amniocentesis begins. Your abdomen is cleaned with an antiseptic. Then, guided by ultrasound images, your doctor inserts a thin, hollow needle through your abdomen and into your uterus. About 2 to 4 teaspoons of amniotic fluid are withdrawn into a syringe. The procedure is over when the needle is removed.

Many women find that the procedure isn't as painful as they had anticipated. You'll notice a stinging sensation or a prick when the needle enters your skin and some menstrual-like cramping during the procedure. The discomfort is usually about the same as having blood drawn.

The sample is sent to a laboratory. Some results may be available within a few hours or days. Chromosomal assessment may take seven to 14 days, because the fetal cells must be allowed to multiply until there are enough to be tested. For information on a rapid 24- to 36-hour preliminary test result, see "FISH: Speeding up genetic analysis," on page 304.

What the results may tell you

Genetic amniocentesis is often used to identify various genetic defects. Some of the information an amniocentesis may provide includes:

• Chromosomal abnormality. Amniocentesis allows the lab to look at the number and structure of each of your baby's 23 pairs of chromosomes. It allows your doctor to check for chromosomal abnormalities such as Down syndrome, trisomy 13 and trisomy 18.

• Neural tube defects. The sample of amniotic fluid can be assessed for abnormally high levels of alpha-fetoprotein (AFP). Elevated levels of AFP may indicate a neural tube defect, such as spina bifida.

Amniocentesis is a definitive diagnostic test for neural tube defects, although the development of advanced ultrasound offers a noninvasive alternative.

• Genetic disorders. The genetic material from cells collected during amniocentesis can be examined for many hereditary disorders. These conditions are relatively rare. They include defects in the body's chemistry (metabolic disorders) such as cystic fibrosis, Tay-Sachs and sickle cell disease; disorders that are passed by the mother to a male infant (X-linked disorders), such as some types of muscular dystrophy and hemophilia; and disorders that are passed from either the mother or father to the baby, such as Huntington's disease. There are so many genetic disorders that it's not practical to look for all of them. These diseases are assessed only if you have a specific reason to look for the problem, such as a family history of a particular genetic disorder.

Maturity amniocentesis is used to determine your baby's lung maturity: • Lung maturity. Testing the amniotic fluid can tell your doctor whether your baby's lungs are developed enough to function outside of the uterus. This is important if you need to deliver the baby early.

Other, less common uses of amniocentesis include:

• Rh incompatibility. If you don't have a type of protein called Rhesus (Rh) factor in your blood (that is, you're Rh negative) but your baby does have it (he or she is Rh positive), you have a condition called Rh incompatibility. Although it's not likely to cause a problem during your first pregnancy, it's possible that your immune system may produce antibodies against the Rh factor in your baby's blood in subsequent pregnancies. This can lead to mild or severe damage or death. Amniocentesis can be used to determine whether and how much your baby is affected.

• Intrauterine infections. In order to determine whether you have an infection caused by a viral agent or parasite such as toxoplasmosis, your doctor may require a sample of the amniotic fluid for analysis.

Possible concerns

Although amniocentesis is a relatively safe test, it carries a few risks:

Miscarriage. Amniocentesis done before 24 weeks of gestation carries a risk of miscarriage of about one in 200 (0.5 percent). Amniocentesis performed early in the pregnancy, before 14 weeks, carries a risk of miscarriage of about two to five in 100 ( 2 percent to 5 percent). Most of these losses occur because of rupture of the amniotic sac. When amniocente-sis is used later in pregnancy to assess lung maturity, a rupture of the amniotic sac is much less likely to cause a fatal complication to the baby because it may be safe to deliver the baby at that point.

• Post-procedural complications. You may have cramping, bleeding or leaking of amniotic fluid after the procedure. Bleeding occurs in 2 percent to 3 percent of cases. Amniotic fluid leakage occurs in about 1 percent of cases. These problems usually go away with no treatment, but call your health care provider if you have bleeding or leakage. Infection rarely occurs, but if you develop a fever following an amniocentesis, contact your health care provider.

• Rh sensitization. In a few cases, amniocentesis may cause an influx of fetal blood into the maternal bloodstream. If this happens and your blood type is Rh negative and your baby's is Rh positive, it may lead to Rh disease. Rh disease can be fatal to the baby. Generally, if you have a negative blood type, you're given a drug called Rh immunoglobulin (RhIg) after the procedure, which can prevent the condition.

• Needle injury. There's a slight chance that the baby may be punctured by the needle, though use of the ultrasound for guidance makes this rare.

Reasons to have it done

The most common reason women have an amniocentesis is their age. If you'll be 35 years or older when your baby is born, your baby has an increased risk of chromosomal abnormalities. As with most prenatal tests, this test is voluntary. The decision to have a genetic amniocentesis is a serious one. Talk to your health care provider or a genetic counselor about your options, no matter what your age. Other reasons you and your health care provider may consider genetic amniocentesis include:

• A previous pregnancy complicated by a chromosomal abnormality or neural tube defect.

• Abnormal results from a screening test, such as a triple test.

• Either parent carries a chromosome rearrangement that doesn't immediately affect him or her but may affect the child.

• Either parent has a central nervous system defect such as spina bifida or a close relative with such a problem.

• Either parent has a chromosomal abnormality such as Down syndrome or a close relative with such a problem.

• Parents are known carriers of a genetic mutation that causes a disease such as cystic fibrosis, Tay-Sachs disease or another single-gene disorder.

• The mother has a male relative with muscular dystrophy, hemophilia or some other X-linked disorder.

What happens next

Most tests come back normal. If there is a problem, you and your health care provider or genetic counselor need to carefully discuss the next step.

Chromosomal problems can't be corrected, and very few treatments for hereditary disorders can be done before birth. This can be a difficult time. It's important to seek out and receive support from your medical team, your family, your spiritual advisers and any others whom you trust and who care about you.

Terminating a pregnancy is never an easy decision. The fact that your child may have a serious or even fatal condition doesn't make that choice any less difficult. Many women decide to continue the pregnancy, and their health care team can refer them to medical and child-care specialists who can help them plan for the future. Adoption is another option. Organizations that specialize in adoption of children with special needs are readily available.

Accuracy and limitations of the test

Although amniocentesis is accurate in identifying certain genetic disorders, such as Down syndrome, it can't identify all birth defects. For example, it can't detect a heart defect, clubfoot or cleft lip and palate. A normal result from an amniocentesis may provide reassurance regarding certain congenital problems, but it doesn't guarantee that your baby is free of all defects.

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Confident Kids

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