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What is amniocentesis?
Amniocentesis is a prenatal test that allows you and your practitioner to gather information about your baby's health and development from a sample of your amniotic fluid. (This is the fluid that surrounds the baby in your uterus.) The test is most commonly done when a woman is between 15 and 18 weeks pregnant to determine whether the baby has genetic or chromosomal abnormalities, such as Down syndrome. But not all women choose to have this test because it carries a small risk of miscarriage.
Other reasons that you may need to have amniocentesis include:
• To check on the well being of your baby if you have a blood sensitization, such as Rh sensitization. This is a complex condition that may occur if your blood is a different type than your baby's.
• To determine whether the baby's lungs are mature enough for an early delivery if you appear to be in premature labor or require an early delivery for any reason.
• To diagnose or rule out a uterine infection if your membranes have ruptured prematurely or your practitioner has any other reason for suspecting you have one.
What birth defects can the test detect?
Amniocentesis can identify several hundred genetic disorders, including some of the most common:
• Nearly all chromosomal disorders, including trisomies, such as Down syndrome and Edward's syndrome. The test is more than 99 percent accurate in diagnosing these conditions. The multiple marker and ultrasound may indicate whether your baby is likely to have these problems, but only amniocentesis can tell you for sure.
• Other genetic disorders such as cystic fibrosis, Sickle cell disease, Tay-Sachs disease, or Huntington's disease. Neither the multiple marker or ultrasound will pick up these problems. Only amniocentesis can provide the information needed to diagnose them in the womb.
• Neural tube defects such as spina bifida and anencephaly. If your amnio indicates that your baby is at risk for these problems, you'll have a detailed follow-up ultrasound for a diagnosis.
More than 95 percent of the high-risk women who have amniocentesis receive good news from the results. Only about five percent will be found to have a baby with a problem. Amniocentesis doesn't detect every birth defect, however — a cleft lip or palate, for example, won't show up on the test.
Is this test appropriate for me?
While most centers will do an amnio for any pregnant woman who wants one, this procedure is not offered routinely to all pregnant women because it carries a small risk of miscarriage. It's usually only offered to those who are found to have a somewhat increased risk of having a baby with a chromosomal or genetic defect. You may be in this category if:
• You're going to be 35 years old or older when your baby is born. The risk of having a child with a chromosomal defect increases as a woman ages. The chance that your baby will have a trisomy disorder like Down syndrome is about one in 250 when you're 35 years old.
• You've had another test, such as a multiple marker screen, that suggested your baby might have a birth defect. If this is the case, you may first have a detailed ultrasound scan to look for physical markers for Down syndrome and other defects. If the ultrasound indicates that there's still cause for concern, you may want to have an amniocentesis for a definitive diagnosis.
• You've already had (or been pregnant with) a child with a birth defect.
• You or your spouse has a family history that indicates your child may be at increased risk of inheriting a genetic disorder. Or you or your spouse is a known carrier of a genetic disorder, such as cystic fibrosis.
• You're taking valproic acid (Depakote) or carbamazipine (Tegretol) for a seizure disorder. These drugs will put your baby at increased risk for spina bifida.
What are the risks of this procedure?
Parents Talk
Have an opinion about this? Add your comments to the bottom of this page.
In deciding whether to have this test, you'll need to weigh your desire to know about your baby's condition with the slight risk that the test could lead to a miscarriage. You'll definitely want to discuss your options with a genetic counselor so that you understand all the risks and benefits of your choices. For example, you may decide to have the multiple marker screen first to gain more information about your risk profile.
There are many reasons to want to know about your child's condition. Even if you know that you would never terminate a pregnancy for any reason, knowing in advance that your baby may have special needs would allow you to prepare for the challenges you'd face. You might want to switch to a better-equipped hospital with specialists. Knowing what's going on with your baby will allow your medical team to monitor your pregnancy as needed and to bring a neonatologist or pediatric surgeon on board to prepare to help your baby when she or he arrives. And some problems can even be treated in utero, such as biotin dependence and MMA (methylmalonic acidemia), both life-threatening body chemistry disorders.
That said, you'll want to weigh the small risk that the procedure may cause you to miscarry. According to the Centers for Disease Control and Prevention (CDC), the risk of miscarriage due to amniocentesis is between one in 200 and one in 400, depending on the skill and experience of the doctor performing it. You'd also have a very slight risk of uterine infection in the days following the procedure (less than one in 1,000), which can sometimes lead to miscarriage. These are all things you should discuss with your partner, your practitioner, and a genetic counselor.
Is there any way to reduce the risks?
The more procedures the physician performing the amniocentesis has done in the past, the lower your risk of complications. If you choose to have amniocentesis, your practitioner or genetic counselor should be able to refer you to an experienced physician. Your best bet is to find someone who does at least 50 of these procedures each year. You'll also want to make sure that an experienced Registered Diagnostic Medical Sonographer provides continuous ultrasound guidance during the procedure. This greatly increases the chances that the physician will be able to obtain enough fluid on the first try and avoid having to repeat the procedure.
What's the procedure like?
You'll most likely have a detailed ultrasound done first to measure the fetus and do an anatomic survey. This may happen on the same day as the amnio or a few days or weeks beforehand. For the amnio, you'll lie on an examining table and have your belly cleaned with alcohol or an iodine solution. You may have someone stay with you throughout the procedure. A doctor or technician will use ultrasound to pinpoint a pocket of amniotic fluid a safe distance from both the baby and the placenta. This part can take up to 20 minutes. Then, under continuous ultrasound guidance, your doctor will insert a long, thin, hollow needle through your abdominal wall and into the sac of fluid around your baby. She'll withdraw a small amount of amniotic fluid — about an ounce, or two tablespoons — and then remove the needle. (The baby will make more fluid to replace what's taken out.) Withdrawing the fluid should take less than 30 seconds.
You may feel some cramping, pinching, or pressure during the procedure. You may opt to have your abdomen numbed first with a local anesthetic, but the pain from the anesthesia injection is likely to be worse than that of the amnio itself and most women decide one needle is enough. The amount of discomfort or pain you may feel varies among women and even from one pregnancy to the next. If you're worried about a needle being that close to your baby, be assured that direct injury to the baby from amniocentesis is very rare. Your doctor will avoid placing the needle near the baby, but if your child happens to come in contact with it, he'll move away quickly just as you would if you bumped into something sharp.
When it's done, your doctor may use an external fetal monitor to listen to the baby's heartbeat for reassurance. You'll want to take it easy for the rest of the day and avoid any heavy lifting for the next two days. You may have some minor cramping for a day or so. One to two percent of women will have significant cramping, ******l spotting, or leaking amniotic fluid. Call your practitioner if you have any of these symptoms or a fever, as there is some chance that they could mean you are about to have a miscarriage. A small amount of fluid leakage is normal, but should be reported anyway.
Note: If your blood is Rh-negative, you'll need a shot of Rh immunoglobulin after amniocentesis since it's possible that your baby's blood may have mixed with yours during the procedure.
When will I get the results?
A laboratory will analyze the fluid sample. First they'll measure the amount of Alpha-fetoprotein (AFP) in the fluid, which will tell you whether there is likely to be an opening in the baby's skin, indicating a neural tube defect, such as spina bifida or anencephaly. They'll also take some of the baby's living cells from the fluid and allow them to reproduce for a week or two. Then they'll test the cells for chromosomal abnormalities or evidence of certain genetic birth defects. You should have the full results within three weeks of the test, although some labs can provide results in as little as eight days.
How do I decide whether to have an amnio or the CVS test?
Both of these tests can tell you if your baby has a chromosomal problem. The CVS test is done earlier in the pregnancy (usually between 10 and 12 weeks) so you could find out sooner about your baby's condition. However, the risk of miscarriage from the CVS is slightly higher. Here are some considerations:
The CVS may be a better choice if: You really need to know your baby's condition while you're still in your first trimester. Some centers may offer amniocentesis during the first trimester, but research indicates that this practice leads to a higher rate of miscarriage than the CVS test and it is considered experimental.
Amniocentesis may be a better choice if: You decide you want to wait for the results of a multiple marker screening (usually done between 15 and 20 weeks), before subjecting yourself to a more invasive test. (The CVS must be done by 12 weeks to be effective.)
