Every mom-to-be hopes to give birth to a healthy baby. However, some women, due to age or genetic makeup, are at greater risk of having a baby with a birth defect. Amniocentesis -- or amnio for short -- is a prenatal diagnostic test that uses a sample of the amniotic fluid that surrounds your baby in the womb to test for specific abnormalities.
Amnio can help rule out many potential problems, including inherited conditions like hemophilia and cystic fibrosis, chromosomal disorders like Down syndrome, and open neural tube defects (a malformation of the brain or spinal cord). The good news is that more than 95 percent of the amnios performed on high-risk moms reveal their babies to be free of the disorder for which they're being tested.
Who should have the test?
Amnio is not offered routinely to all pregnant women. It is considered an invasive procedure and carries a small risk of miscarriage. (Women are considered to have a 1-in-500 to 1-in-300 chance of miscarrying after the procedure, although one study found the miscarriage rate following amnio to be only one in 1,600).
A few years ago, as a result of improvements in less-invasive screening methods, experts changed their guidelines for the procedure. Although in the past, amniocentesis was recommended for women 35 years or older when the baby was conceived or those with a family or partner history of an inherited genetic disorder, the revised guidelines from the American Congress of Obstetricians and Gynecologists recommend that all women be offered a less-invasive screening method (rather than amniocentesis or chorionic villus sampling) before the 20th week of pregnancy.
The ACOG guidelines also offer all women the option of amnio and other diagnostic testing if needed. If the non-invasive screening indicates that the woman is at high risk of delivering a child with a chromosomal disorder, for example, a woman of any age should be able to opt for the more invasive diagnostic procedure.
The association notes that although screening may result in some false positives or negatives, diagnostic testing is thought to pick 100 percent of all gene defects that result in Down syndrome and some other chromosomal abnormalities.
When is amniocentesis performed?
This test is typically done in your second trimester, between the 15th and 20th weeks of pregnancy. (Updated guidelines in 2016 note that the test should not be performed before 15 weeks.) Amnio may also be performed during the third trimester to detect specific problems. If your baby has a blood type that is incompatible with yours, a condition called Rh disease, an amnio can determine whether your baby is anemic and needs a transfusion.
If you need to deliver early (before 37 weeks) or you go into premature labor, an amnio can tell if the baby's lungs are mature enough to function properly. The test may also be used to confirm or rule out a uterine infection, particularly if the fluid-filled amniotic sac around your baby breaks prematurely.
Many hospitals make sure that women getting amnios make an appointment with a genetic counselor before the procedure. This is advisable because there are now first-trimester tests that may help women of any age define their risk of birth defects before getting amniocentesis. The counselor will go over any family history of genetic disease as well as advise women over 35 of their risk of having a baby with birth defects.
Since amniocentesis also reveals the sex of the baby, the counselor will probably ask you if you'd like to know your baby's gender along with your test results. It's a matter of personal preference whether you and your partner want to know before your baby is born.
How is an amniocentesis performed?
Outcomes are best when a highly experienced practitioner performs the test at a major medical center. In most cases, you can bring someone with you for moral support during the procedure. To begin, you'll lie on an examination table and a technician will swab your belly with antiseptic. Then the technician or a doctor will use ultrasound to find a pocket of amniotic fluid a safe distance from both your baby and the placenta, the tissue that nourishes your baby. You'll need to have a full bladder for the ultrasound, so find out beforehand how much water you need to drink and when. This part of the procedure may take as long as 45 minutes.
Once the practitioner has pinpointed a safe spot, she will insert a long, fine, hollow needle through your abdomen and uterus and into the amniotic sac of fluid, all under continual ultrasound guidance. Don't worry -- your baby is extremely unlikely to get pricked by the needle. If by some odd chance the needle does come in contact with your baby, he'll move away quickly, just as you would if you touched something sharp. The technician will withdraw a small amount of fluid, less than an ounce, or about one to two tablespoons, and then slowly remove the needle. The sample will contain live fetal cells that normally float in the amniotic fluid. Your body makes more fluid to replace what's taken.
You may experience cramping, pinching, or pressure during the procedure. You can opt to have your belly numbed first with a local or topical anesthetic, but some women who have experienced it both ways say the pain from the anesthetic injection is worse than the amnio itself. The choice is yours. The amount of pain or discomfort you may feel is an individual thing that can even vary from one pregnancy to the next.
When the procedure is completed, your practitioner may use an external fetal monitor to listen to the baby's heartbeat in order to make sure that all is well. You'll want to take it easy for a few hours and avoid heavy lifting, prolonged standing, or strenuous exercise for the remainder of the day.
What can I expect after the test?
You may have some light cramping for a day or two. Some women -- around 1 to 2 percent -- experience significant cramping, light vaginal bleeding known as spotting, or leaking amniotic fluid. If you experience these symptoms, or develop a fever, call your doctor immediately.
A small amount of fluid leakage is nothing to worry about, but it's safer to report it anyway. For most women it's safe to resume normal activities the day after the amniocentesis. The clinic where you have the amnio should give you clear instructions. If you have any questions when you get home, be sure to call.
How long does it take to get the results?
Your amniotic fluid is sent to a laboratory for analysis. First, the technicians will measure the amount of a chemical called alpha-fetoprotein, or AFP, in the fluid. High levels may indicate the potential for an open neural tube defect such as spina bifida.
Low levels of AFP, in combination with other indicators, may suggest a chromosomal abnormality. If early screening tests or family history suggest a problem is possible, the technicians will also take some of your baby's cells from the fluid and allow them to multiply for a week or two in a cell culture. Then they'll test the cells for abnormalities. Complete results can take as long as three weeks, although some labs may be able to get them out in a little over a week.
What if my results are abnormal?
What happens next depends on what disorder is diagnosed and what you decide to do about the new information you have. Unfortunately, today's healthcare professionals are able to confirm many more birth defects than they're able to treat after birth or even remedy prenatally. If your baby has a condition for which prenatal treatment is available, you'll want to discuss your options with your partner, healthcare team, and/or experts in this area.
If the baby has an ailment for which prenatal treatment is not yet available, you'll also want to discuss your situation with your partner, healthcare providers, genetic counselors, spiritual advisors, or any other support people. In some cases you'll want to prepare emotionally, practically, and financially for the arrival of a baby with a birth defect. In other circumstances, you may decide to discontinue the pregnancy. The choice is a personal one and yours to make, once you've had a chance to consider all the factors involved.
ACOG Issues New Screening Guidelines, Apri 29, 2016
ACOG's Screening Guidelines on Chromosomal Abnormalities What They Mean to Patients and Physicians, American College of Obstetricians and Gynecologists, May 7, 2007.
Olney, R. et al. Chorionic Villus Sampling and Amniocentesis: Recommendations for Prenatal Counseling. Mortality and Morbidity Weekly Report. Centers for Disease Control and Prevention
American College of Obstetricians and Gynecologists. Planning Your Pregnancy and Birth. Third Edition.
The Cleveland Clinic Health Information Center. Genetic Amniocentesis. http://www.clevelandclinic.org/health/health-info/docs/0500/0548.asp?index=4206
Johns Hopkins University. Genetics and Public Policy Center. Prenatal Genetic Testing. http://www.dnapolicy.org/genetics/prenatal.jhtml
National Institute of Child Health & Human Development. Prenatal Diagnosis. http://www.nichd.nih.gov/publications/pubs/mrdd/sub10.htm
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American College of Obstetricians and Gynecologists. Special Care. Genetic Defects Disorders and Birth.
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National Hemophilia Foundation. Inheritance of Hemophilia Fact Sheet. http://www.hemophilia.org/resources/inheritance.htm
March of Dimes. Amniocentesis. http://www.marchofdimes.com/professionals/14332_1164.asp