What is amniocentesis?
Amniocentesis is a prenatal test that allows your healthcare practitioner to gather information about your baby's health from a sample of your amniotic fluid. This is the fluid that surrounds your baby in the uterus.
The most common reason to have an "amnio" is to determine whether a baby has certain genetic disorders or a chromosomal abnormality, such as Down syndrome.
Just like chorionic villus sampling (CVS), a procedure done in the first trimester, amniocentesis produces a karyotype – a picture of your baby's chromosomes – so that your caregiver can see for sure if there are problems. (In about 1 percent of cases, there's a problem with the specimen and the test doesn't yield a result.)
Amniocentesis is usually done when a woman is between 16 and 20 weeks pregnant. Though all women should be offered the option of having an amnio, women who choose to have this test are often 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 other reasons that amniocentesis may be done:
- To diagnose or rule out an intrauterine infection
- 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 assess the maturity of your baby's lungs when considering an early delivery for medical reasons (unless it's clear the baby needs to be delivered immediately regardless of lung maturity)
What disorders and defects can amniocentesis detect?
Amniocentesis is used to detect:
- Nearly all chromosomal disorders, including Down syndrome, trisomy 13, trisomy 18, and sex chromosome abnormalities (such as Turner syndrome). The test can diagnose these conditions, but it can't measure their severity.
- Several hundred 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, amniocentesis can usually tell you whether he has the disease.
- Neural tube defects such as spina bifida and anencephaly, which is done by measuring the level of a substance called alpha-fetoprotein (AFP) in the amniotic fluid.
An amniocentesis cannot, however, detect other structural birth defects — such as heart malformations or a cleft lip or palate, for example. That said, many structural defects can be picked up on the second-trimester ultrasound that's routinely done for every woman having an amnio.
What factors would put me at increased risk of having a baby with a genetic abnormality or disorder?
Some factors to consider include:
Down syndrome screening results
You had a screening test that indicated your baby is at a higher-than-average risk for Down syndrome or another chromosomal problem.
An ultrasound showed that your baby has structural defects associated with a chromosomal problem.
Carrier screening results
You and your spouse are carriers of a recessive genetic disorder such as cystic fibrosis or sickle cell disease.
You've previously been pregnant with a child with certain genetic abnormalities and may have a higher risk of this happening again.
Your family history
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.
Anyone can have a baby with a chromosomal abnormality, but the risk rises with the mother's age. For example, your likelihood of carrying a baby with Down syndrome rises from about1 in 1,200 at age 25 to 1 in 100 at age 40.
What is the risk of miscarriage from amniocentesis?
The risk of miscarriage due to amniocentesis is low. 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.
Estimates vary, but according to the American College of Obstetricians and Gynecologists (ACOG), the loss rate from the procedure is as low as 1 in 300 to 500 — and perhaps even lower if the doctor or center has a lot of experience performing amnios.
Is there any way to reduce the risk?
Ask your practitioner or genetic counselor to refer you to a doctor with plenty of experience or a center that performs a lot of amnios. 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 greatly improves the chances that the doctor will be able to obtain enough fluid on the first try, so you can 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 genetic counselor before deciding whether to have an amnio?
Yes. In fact, most testing centers require that you meet with a genetic counselor to discuss the risks and benefits of various methods of prenatal screening and testing before you have an invasive procedure like amniocentesis or CVS. 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 CVS or amnio, or skip the testing altogether.
How can I decide what's right for me?
ACOG 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 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, you can then decide — with the help of your practitioner or genetic counselor — whether your results indicate a high enough risk that you want to have CVS or amnio to determine whether a problem exists. You'll need to weigh your need to know about your baby's condition against the small chance that diagnostic testing could cause a miscarriage.
Some women who opt for invasive testing are pretty clear 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 may want to switch to a better-equipped hospital with specialists.
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. So they decide not to have invasive testing.
There's no one right decision. Individual parents-to-be have different feelings about what risks are acceptable 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 10 and 13 weeks), so you can find out sooner about your baby's condition. If everything's okay, your mind will be put at ease that much sooner. Or, if there is a serious problem and you opt to terminate the pregnancy, you'll be able to do so while you're still in the first trimester.
On the other hand, you may prefer to wait for the results from second-trimester screening before subjecting yourself to an invasive test. At that point, amniocentesis would be your only option.
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. (Although they can usually be detected with a mid-pregnancy ultrasound instead.)
CVS has been thought to have a slightly higher miscarriage rate than amniocentesis, but recent research suggests this is not the case. Experienced doctors and medical centers that perform a lot of these procedures may have similar low miscarriage rates for both.
Before making a decision, you'll want to discuss all of these issues with your partner, your health practitioner, and possibly a genetic counselor.
What's amniocentesis like?
The procedure usually takes about 20 to 30 minutes. Before you have your amnio, an ultrasound is done to measure your baby and check his basic anatomy. (Some testing centers do this when you come in for your amnio. Others do it in advance.)
For the amnio itself, you lie on an examining table and your belly is cleaned with alcohol or an iodine solution to minimize the risk of infection. Ultrasound is used 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. The doctor withdraws a small amount of amniotic fluid — about an ounce, or two tablespoons — and then removes 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.
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 immune globulin after amniocentesis unless the baby's father is Rh-negative as well. (Your baby's blood may mix 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 intercourse for about a week and strenuous activities and air travel for the next two or three days. (Flying isn't risky, but it's a good idea to stay close to home for a few days in case you have any symptoms that need to be checked out.)
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. It could be a sign 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 in one to two weeks.
During this period, a laboratory analyzes the fluid sample, measuring the amount of alpha-fetoprotein (AFP) in the fluid. 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 checks the cells for chromosomal abnormalities and evidence of certain genetic birth defects. (You can also find out your baby's sex if you want to.)
In certain circumstances, you may 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. These 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 women opt to terminate the pregnancy, while others 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.
Join BabyCenter's online Amnio Support & Information Group.