Amniocentesis

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 your baby in the uterus. The test is most commonly done when a woman is between 15 and 20 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, in part 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 can occur if your blood is a different type than your baby’s.
  • To determine whether your 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 water has broken prematurely or your practitioner has any other reason for suspecting you have one.
  • Which birth defects can the test detect?

    Amniocentesis can identify several hundred genetic disorders, including some of the most common:

    • Nearly all chromosomal disorders, including Down syndrome and Edward’s syndrome. The test is more than 99 percent accurate in diagnosing these conditions. An ultrasound, a combined first-trimester screening, or a multiple marker test 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, and Huntington’s disease. Neither the multiple marker test nor 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.

    More than 95 percent of the high-risk women who have amniocentesis receive good news from the results. Only about 5 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?

    Although most centers will do an amnio for any pregnant woman who wants one, the procedure isn’t offered routinely to all pregnant women because it carries a small risk of miscarriage. It’s usually only offered to those 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’ll be 35 years old or older on your due date. The risk of having a child with a chromosomal defect rises as a woman ages. The chance that you’re carrying a baby with Down syndrome, for instance, is about one in 270 when you’re 35 years old — compared to 1 in 1,250 when you’re 25.
    • You’ve had another test, such as a multiple marker screen, a nuchal fold scan, or a "combined screening," that indicated your baby is at a higher risk for a problem. If this is the case, you may first have a detailed ultrasound scan to look for physical signs of Down syndrome and other defects. If the ultrasound shows that there’s still cause for concern, you may want to have an amniocentesis for a definitive diagnosis.
    • You’ve previously been pregnant with a child with a birth defect.
    • You or your partner has a chromosomal abnormality or genetic disorder or a family history that puts your child at increased risk for genetic problems. Or you and your spouse are both carriers of a recessive genetic disorder such as cystic fibrosis or sickle cell disease.
    • What are the risks of the procedure?

      In deciding whether to have this test, you’ll need to weigh your desire to know about your baby’s condition against the slight risk that the test will 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 a combined first-trimester screening or a multiple marker test first to gain more information about your risk for certain problems.

      There are many reasons to want to know about your child’s condition. Even if you would never terminate a pregnancy for any reason, knowing in advance that your baby will have special needs allows you to prepare for the challenges you might face. You might want to switch to a better-equipped hospital with specialists, for example. Knowing what’s going on with your baby allows 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 after delivery. And a few problems can even be treated in utero, such as biotin dependence and MMA (methylmalonic acidemia), both life-threatening, though extremely rare, body chemistry disorders.

      That said, you’ll want to consider 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 from 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 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 times the physician who does the amnio has performed the procedure, the lower your risk of complications. Ask your practitioner or genetic counselor to refer you to a physician with plenty of experience, preferably one who does at least 50 amnios a 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. And when continuous ultrasound guidance is used, injuries to the baby from the amnio needle are very rare.

      Can I meet with a counselor before deciding whether to have the test?

      Most testing centers require that you meet with a genetic counselor to discuss the risks and benefits of various methods of prenatal testing before you have the test. The counselor will take down your family history and ask questions about your pregnancy. Your answers will enable the counselor to give you a sense of your risk for a problem and determine whether you need to be screened for a particular genetic disease. Then you can decide whether you want to go ahead with the amnio.

      What’s the procedure like?

      Before having your amnio, you’ll have an ultrasound to measure your baby and check his basic anatomy. This may happen on the same day as the amnio or a few days or weeks beforehand.

      For the amnio itself, you lie on an examining table and your belly is cleaned with alcohol or an iodine solution. (Someone may stay with you throughout the procedure.) A doctor or technician uses 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, the doctor inserts a long, thin, hollow needle through your abdominal wall and into the sac of fluid around your baby. She withdraws a small amount of amniotic fluid — about an ounce, or two tablespoons — and then re moves the needle. Withdrawing the fluid should take less than 30 seconds. And your baby will make more fluid to replace what’s taken out.

      You may feel some cramping, pinching, or pressure during the procedure — or you may feel no discomfort at all. The amount of discomfort or pain varies among women and even from one pregnancy to the next. You may choose 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 that one needle is enough.

      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 with continuous ultrasound guidance. 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.

      Afterward, 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. About 1 to 2 percent of women will have significant cramping, vaginal spotting, or leaking amniotic fluid. Call your practitioner if you have any of these symptoms — or a fever — because they could be signs of impending miscarriage.

      Note: If your blood is Rh-negative, you’ll need a shot of Rh immunoglobulin after amniocentesis (unless the baby’s father is Rh-negative as well) because it’s possible that your baby’s blood mixed with yours during the procedure.

      When will I get the results?

      You should have the full results within two weeks, although you can sometimes get results in as few as eight days. During this waiting period, a laboratory analyzes the fluid sample, first measuring the amount of alpha-fetoprotein (AFP) in the fluid. This shows whether there’s likely to be an opening in the baby’s skin, indicating a neural tube defect such as spina bifida or anencephaly. The lab also takes some of the baby’s living cells from the fluid and allows them to reproduce for a week or two, then tests the cells for chromosomal abnormalities and evidence of certain genetic birth defects.

      How do I decide whether to have an amnio or the CVS test?

      Both these tests can tell you whether your baby has a chromosomal problem. The CVS test is done earlier in pregnancy (usually between 10 and 12 weeks), so you can find out sooner about your baby’s condition. Some centers offer amniocentesis during the first trimester, but research indicates that this practice leads to a higher rate of miscarriage and it’s considered experimental. Here are some considerations:

      The CVS may be a better choice if you want to know your baby’s condition while you’re still in your first trimester. You may have heard that the risk of miscarriage from the CVS is slightly higher than from amniocentesis, but the most recent research has found this to be untrue.

      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.)

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