What is Amniocentesis?
Amniocentesis is a prenatal test that allows your healthcare practitioner to gather information about your baby's health and development from a sample of the amniotic fluid. This is the fluid that surrounds your baby in the uterus.
Amniocentesis is usually done when a woman is between 16 and 20 weeks pregnant. Women who choose to have this test are primarily those at increased risk for genetic and chromosomal problems, in part because the test is invasive and carries a small risk of miscarriage.
Here are a few reasons that amniocentesis may be done.
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, for instance, your water has broken prematurely.
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. (Note: Obstetricians are increasingly using Doppler ultrasound for this purpose instead of amnio.)
To detect chromosomal disorders, including Down syndrome, trisomy 13, trisomy 18, and sex chromosome abnormalities (such as Turner syndrome and Klinefelter syndrome). Amniocentesis is more than 99 percent accurate in diagnosing these conditions.
To detect genetic disorders, such as cystic fibrosis, sickle cell disease, and Tay-Sachs disease. The test is not used to look for all of them, but if your baby is at increased risk for one or more of these disorders, in most cases amniocentesis can usually tell you whether she or he has the disease.
To detect neural tube defects such as spina bifida and anencephaly.
Amniocentesis does not detect birth defects such as heart malformations or a cleft lip or palate. However, many structural defects can be picked up on the second-trimester ultrasound that's routinely done for every woman having an amnio.
What is the risk of miscarriage from amniocentesis?
There is no real consensus on the actual risk of miscarriage due to amniocentesis, however, it is estimated to be between one in 200 and one in 400. One recent study estimated the risk to be as low as one in 1,666, although some experts dispute the methodology used to arrive at this figure.
Because a certain percentage of women will end up miscarrying in the second trimester anyway, there's no way of knowing for sure whether a miscarriage following an amnio was actually caused by the procedure. Your particular risk depends in large part on the skill and experience of the doctor performing the procedure.
Is there any way to reduce the risk?
Ask your practitioner or genetic counselor to refer you to a doctor with plenty of experience, preferably one who does at least 50 amnio's a year. And ask about the estimated procedure-related miscarriage rate of the doctor or the center where you're considering having the procedure done.
You'll also want to make sure that an experienced registered diagnostic medical sonographer provides continuous ultrasound guidance during the procedure. This will improve the chances that the doctor will obtain enough fluid on the first try and avoid a repeat. And, when continuous ultrasound guidance is used, injuries to the baby from amnio needle are very rare.
Can I meet with a genetic counselor before deciding whether to have an amnio?
Yes. Most testing centers require you meet with a genetic counselor to discuss risks and benefits of various methods of prenatal screening and testing before you have an invasive procedure like amniocentesis. 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 having a baby with chromosomal problems or a particular genetic disease. Then you can decide whether you want to be screened, go right for amnio or forgo testing altogether.
How can I decide what's right for me?
The American College of Obstetricians and Gynecologists now recommends that women of all ages be offered first- and second-trimester screening and diagnostic testing options. Your practitioner or genetic counselor should discuss the pros and cons of the available approaches with you. But ultimately, whether or not to test is a personal decision.
Many women opt for screening and then make a decision about diagnostic testing based on the initial results. Other women opt for diagnostic testing right away. (They may know that they're at high risk for a chromosomal problem or a condition that can't be detected by screening or they may just feel that they want to know as much as possible about their baby's condition and are willing to live with the small risk of miscarriage to find out.) Some women decide to have no screening or testing.
If you opt for screening first, your results with the help of your practitioner or genetic counselor will indicate a risk level after which you may elect an amnio. Some women who opt for invasive testing are pretty sure in advance that they would terminate the pregnancy if a serious problem were found. Others may decide to have an amnio even though they aren't sure what they'd do if they got bad news.
Some women may decide to have an amnio even if they know they would never terminate a pregnancy. They may feel that finding out that their baby will have special needs will help them prepare emotionally for the challenges ahead. In certain cases, they will want to switch to a better-equipped hospital for post-birth support.
There are a few conditions that may be treated while the baby is still in the womb. So if there's a strong suspicion that your baby may have one of these rare problems, you might opt for an amnio to find out. On the other hand, some women feel strongly about not having a procedure that increases their risk of miscarriage, especially if the results aren't going to affect the management of their pregnancy.
There's no one right decision. Individual parents have different feelings about acceptable risks and may arrive at different conclusions when facing the same set of circumstances.
If I choose diagnostic testing, how do I decide between amnio and CVS?
Both tests can tell you whether your baby has a chromosomal problem or certain genetic disorders. CVS is done earlier in pregnancy (usually between 11 and 12 weeks), so you can find out sooner about your baby's condition.
Other considerations may influence your decision as well. For example, if you're at high risk for having a baby with a neural tube defect, you may want to have amniocentesis, as CVS cannot diagnose these defects.
What's amniocentesis like?
Before you have your amnio, you'll have an ultrasound to measure your baby and check his basic anatomy. (Some testing centers do this when you come in for your amnio, while others do it in advance.)
For the amnio itself, you'll lie on an examining table and your belly will be cleaned with alcohol or an iodine solution. Ultrasound is used to pinpoint a pocket of amniotic fluid a safe distance from both the baby and the placenta. This can take up to 20 minutes.
Then, under continuous ultrasound guidance, the 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.
Withdrawing the fluid can take a few minutes but usually takes less than 30 seconds. 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 moms-to-be 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.
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 your baby's blood may have mixed with yours during the procedure and it may not be compatible.
What happens after the procedure?
You'll need to take it easy for the rest of the day, so arrange for someone to drive you home. Avoid any heavy lifting, intercourse, and air travel for the next two or three days.
You may have some minor cramping for a day or so. If you have significant cramping or vaginal spotting, or you're leaking amniotic fluid, call your practitioner immediately. They could be signs of impending miscarriage. Also call right away if you have a fever, which could be a sign of an infection.
When will I get the results?
You should have the full results within two weeks. During this 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. (You can also find out your baby's gender if you want to.)
Sometimes you can get preliminary results while you're waiting for the cells to reproduce. For example, in some cases, a technique called fluorescence in situ hybridization (FISH) may be used to look for select problems. Preliminary results are available quickly, often in a couple of days.
What happens if my baby is found to have a problem?
You will be offered genetic counseling so that you can get more information and discuss your options. Some may opt to terminate the pregnancy, while others will decide to continue.
Whichever route you choose, you may find that you want further counseling or support. Some women find support groups helpful, others may want individual counseling, and some may choose both. Be sure to let your practitioner and genetic counselor know if you need more help so they can give you the appropriate referrals.