Amniocentesis ("amnio" for short) is the most accurate test available for detecting a variety of genetic disorders and birth defects such as Down syndrome, as well as neural tube defects. Amniocentesis is called a prenatal diagnostic test because it can tell you, with as much certainty as possible, whether your baby has a genetic disorder. It analyzes the genes from fetal cells that are found in amniotic fluid. Prenatal screening tests, including blood tests and ultrasound, can only tell you if your baby has a higher risk of a disorder. If you have positive results on screening tests, your doctor may recommend having amniocentesis or CVS, the other type of diagnostic test, so that you can know for sure. It’s helpful to remember that the vast majority of women have a normal result and a healthy baby.
It is usually performed between the 15th and 20th weeks of pregnancy. Having the test before 14 weeks could be harmful to a developing baby. Depending on the way the cells are analyzed and which disorders you are testing for, you may get the results within a day or up to several weeks.
Amniocentesis tests for abnormalities in genes or chromosomes. It can detect disorders (known as aneuploidy) that are caused by either an an extra chromosome (called trisomy), such as Down syndrome, or a missing chromosome (called monosomy), such as Turner syndrome. It can also diagnose several hundred Inherited disorders, such as sickle cell disease, cystic fibrosis, and Tay-Sachs disease, which are caused by a mutation in a gene. In most cases, both parents must carry the same mutated gene in order for their baby to inherit the disorder. Amniocentesis won’t look for every possible disease, but if your baby is at an increased risk for one or more, the test can usually detect it.
Amniocentesis can also detect neural tube defects, such as spina bifida, by measuring levels of alpha-fetoprotein in the amniotic fluid. Neural tube defects occur when the developing spine doesn’t close properly. Alpha-fetoprotein is produced by the baby’s liver, and more of it leaks out into the amniotic fluid if there is an opening in the spine.
Because ultrasound is used during an amniocentesis, it can detect some other types of birth defects, such as cleft palate and heart defects, that are not detected by amniocentesis.
It is more than 99% accurate in detecting genetic disorders, and more than 90% accurate in diagnosing neural tube defects. Amniocentesis can’t diagnose all birth defects and genetic diseases because some conditions can’t yet be detected prenatally. Many heart defects go unnoticed until after birth, although they may be detected by a comprehensive ultrasound at a center that does a high volume of scans.
Doctors and genetic counselors generally recommend the test for any woman who is at increased risk of having a baby with a chromosomal abnormality or neural tube defect. All women over age 35 used to be advised to consider an amniocentesis, but first- and second-trimester screening blood tests and ultrasound are more accurate than they used to be, and doctors may take those results into account when advising women of all ages.
Amniocentesis is usually considered for women who:
Some women who don’t meet any of these criteria choose to have an amnio for their own peace of mind. Amniocentesis also reveals the sex of the baby, but it is never given just for this reason (and your doctor can keep the sex a secret if you'd rather be surprised).
Recent research has found that the risk is much lower than doctors once thought. Although you may still hear that the risk of having a miscarriage after an amniocentesis is between 1/200 and 1/400, those statistics were based on studies done in the 1970s, when ultrasound was not always used to make sure doctors guide the needle safely. Experts now say that the risk is between 1/500 and 1/1000, and the biggest study that was done with 35,000 women around the country found that the risk was only 1/1600.
You can reduce the risk by having your amniocentesis done by a doctor who has a lot of experience or at a center that performs many amnios and uses a registered diagnostic medical sonographer.
Amniocentesis can be performed by a general ob/gyn or a perinatologist, who specializes in maternal-fetal medicine.The first step of the test is an ultrasound, which is used to measure the baby and ensure that you're at least 14 weeks pregnant. Then an antibacterial iodine solution is rubbed onto your abdomen. Next, using the ultrasound transducer to get an image of the baby's position, the doctor inserts a thin, hollow needle through your abdominal wall and into your uterus. Navigating carefully by using the image on the ultrasound monitor, the doctor withdraws about an ounce of amniotic fluid. (Don’t worry—you’ll make more quickly.) The needle is then withdrawn, and the sample is sent to the lab for analysis. It may sound scary, but the procedure takes a minute or two and shouldn't be more painful than having blood drawn.
In the lab, fetal cells in the amniotic fluid are grown and cultured and the chromosomes are analyzed for abnormalities. Couples who are carriers of a specific genetic disease, such as Tay-Sachs or cystic fibrosis, will receive a more targeted analysis.
Yes, but you should avoid strenuous exercise, heavy lifting, and sexual activity for two to three days. You may have menstrual-like cramps for a few hours, a small leakage of amniotic fluid, or discomfort around the needle site. Call your doctor if any of these side effects continue or worsen, or if you have a fever.
Your health insurance should cover the procedure if your doctor recommends it, although you may have to pay a portion of the cost. Insurance may not cover the test if you’re considered to be at low risk.
Probably the hardest part of having an amniocentesis is waiting for the results. If the lab's results indicate a problem, your doctor and a genetic counselor will explain the results and offer guidance about your options. You can also consult with a pediatric or surgical specialist who can help you understand more about the disorder, treatment options, and future issues you may face. It may be possible to have additional testing, such as a specialized ultrasound exam, to find out more detail about a defect. Expectant parents may face agonizing decisions at a relatively late stage of pregnancy, and need to consider physical, psychological, emotional, and financial issues.
Chorionic villus sampling (CVS) is a diagnostic test that evaluates cells from the placenta that contain the same genes as the fetus. The advantage of CVS is that it is done in the first trimester, typically between 10 and 13 weeks, and results are usually ready sooner. With an experienced doctor, the risk of miscarriage is about the same as for amniocentesis. Although CVS can detect genetic disorders and chromosomal abnormalties, it can’t provide information about neural tube defects. If you have CVS, you’ll still need a blood test at 16 to 18 weeks to screen for neural tube defects and may be advised to have a follow-up amniocentesis.
Deciding whether to have an amniocentesis can be one of the most difficult decisions to make during pregnancy. There is no right or wrong choice. It can be helpful to think about what you would do if the results come back positive. You may know that you’d want to terminate the pregnancy if a serious problem were found, or you might not be sure. If you're certain you would never terminate a pregnancy for any reason, you should discuss your feelings with your healthcare provider. He or she can help you decide if having amniocentesis would be worthwhile. Knowing in advance that your baby will have special needs gives you time to prepare, get additional support, and perhaps arrange to deliver at a high-risk tertiary care hospital. Some conditions can now even be treated while the baby is in the womb.