Amniocentesis

What is amniocentesis?

Amniocentesis is a prenatal test that's done to determine whether a baby has certain genetic disorders or a chromosomal abnormality, such as Down syndrome. It's usually done between 15 and 20 weeks of pregnancy, but it can be done any time after as well.

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 your doctor can see whether there are abnormalities.

The test requires your healthcare practitioner to use a needle and ultrasound to take a small sample of your amniotic fluid, the fluid that surrounds your baby in the uterus. Amnio carries a very small risk of miscarriage. 

Which 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 isn't 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 they have the disease.

An amniocentesis can't detect structural birth defects such as heart malformations or a cleft lip or palate. Many structural defects can be picked up on the second-trimester ultrasound that's routinely done for every woman.

Why would I have amniocentesis?

All women should be offered the option of having amniocentesis. But women who choose to have this test are often those at increased risk for genetic and chromosomal problems.

Here are some reasons you might choose to have an amnio:

Down syndrome screening results
You had a first-trimester screening that indicated your baby is at a higher risk for Down syndrome or another chromosomal problem. First-trimester screenings include the nuchal translucency scan (NT scan) and noninvasive prenatal testing, or NIPT.

Ultrasound results
An ultrasound showed that your baby has structural defects associated with a chromosomal problem.

Carrier screening results
You and your partner are carriers of a recessive genetic disorder such as cystic fibrosis or sickle cell disease.

Your history
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 have a chromosomal abnormality or genetic disorder, or a family history that puts your child at increased risk for genetic problems.

Your age
Anyone can have a baby with a chromosomal abnormality, but the risk increases with the mother's age. For example, your likelihood of carrying a baby with Down syndrome rises from about 1 in 1,200 at age 25 to 1 in 100 at age 40.

What are the risks of amniocentesis?

The amniocentesis risk that usually concerns women most is miscarriage. Estimates vary, but according to the American College of Obstetricians and Gynecologists (ACOG)Opens a new window, the chance of miscarriage from the procedure is about 1 in 900.

Because a certain percentage of women will end up miscarrying in the second trimester, there's no way of knowing for sure whether a miscarriage following an amnio was actually caused by the procedure.

Other risks include passing an infection (such as HIV or hepatitis) from you to your baby and leaking amniotic fluid. The leaking amniotic fluid is usually a small amount (that your body replaces) and stops on its own within a week.

To reduce the risks:

• Ask your practitioner or genetic counselor to refer you to a doctor with plenty of experience or a center that performs amnios regularly.
• Ask about the estimated procedure-related miscarriage rate of the doctor or the center where you're considering having the amnio done.
• 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.

How can I decide whether to have amniocentesis?

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.

ACOG recommends that women of all ages be offered genetic 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.

Options include:

• No screening or testing. Some women decide to forgo all screening and testing because the results wouldn't influence their pregnancy. (They wouldn't terminate the pregnancy if a serious problem were found.) Some women aren't willing to take even a very small risk of miscarriage in order to find out the results of diagnostic testing.
• Screening. Many women opt for screening and then make a decision about diagnostic testing based on the initial results. 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 amnio or CVS to determine whether a problem exists. 
• Diagnostic testing. Some women opt for diagnostic testing after screening, and some choose diagnostic testing right away. Women who choose diagnostic testing right away 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. There are even 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 amnio to find out.

Some women who opt for invasive testing are pretty clear in advance that they would have an abortion if a serious problem were found. Others 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'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.

How is amniocentesis done?

The procedure usually takes about 20 to 30 minutes. Here are the steps:

1.      An ultrasound is done to measure your baby and check their basic anatomy. (Some testing centers do this when you come in for your amnio. Others do it in advance.)

2.      You lie on an examining table and your belly is cleaned with alcohol or an iodine solution to minimize the risk of infection.

3.      Ultrasound is used to pinpoint a pocket of amniotic fluid a safe distance from both the baby and the placenta.

4.      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.

5.      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.

6.      Your doctor will check your baby's heartbeat using the ultrasound for reassurance.

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.

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 amniocentesis?

You'll need to take it easy for the rest of the day, so arrange for someone to drive you home.

Also:

• Avoid sex and strenuous activities for the next two or three days.
• Don't plan any travel. 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.
• Call your doctor or midwife if you have unusual symptoms. Some minor cramping for a day or so is normal. But 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 amniocentesis results?

Some of your preliminary test results may be available in a couple of days. In some cases, a technique called fluorescence in situ hybridization (FISH) may be used to look for select problems. These results are available quickly.

Some of the testing can take several weeks, though. 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.)

Ask your doctor about receiving early results as they become available.

What happens if my baby has a problem?

You'll be offered genetic counseling and a consultation with a maternal fetal medicine specialist so 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.