To the Point– Amniocentesis is a diagnostic test that can tell you with very high accuracy if your baby has a chromosome abnormality such as Down syndrome, by looking directly at the fetal chromosomes. The test is performed by inserting a very thin needle into the sac of fluid surrounding the baby (amniotic sac) using ultrasound guidance and removing a small amount of fluid. This fluid contains cells from the baby which are sent to the lab. Amniocentesis is generally considered safe but not entirely risk-free; there is a risk of less than 1 in 200 (0.5%) for miscarriage associated with the procedure. The procedure is typically done at 15-20 weeks gestation.
Amniocentesis is a diagnostic test, meaning the results are typically “yes or no”. The procedure itself is usually performed between 15-20 weeks gestation. The test is performed by inserting a very thin needle into the sac of fluid surrounding the baby (amniotic sac) using sonographic guidance and drawing out a small amount of fluid. The fluid contains skin cells from the fetus that have naturally fallen off (just like you lose skin cells every day, so does the fetus) and bladder cells. Each of these cells has a set of chromosomes (see prenatal genetics 101 for more info). With amniocentesis, the lab can open up the cells and look directly at the baby’s chromosomes.
Amniocentesis can be done in the doctor’s office. Most women report that it can be uncomfortable, but most generally do not experience considerable pain. There is a risk of miscarriage, which for a long time was thought to be approximately 1/200. New data suggests that it is lower than that. Ultimately, over 99% of the time everything goes just fine. If you do have an amniocentesis, you need to take it easy for 24-48 hours – ask your doctor for details. You will get results on all of the chromosomes, including the sex chromosomes, which will tell you if it is a boy or a girl .
Amniocentesis does not look for or rule out all birth defects or genetic conditions, but it does rule out age-related chromosome abnormalities such as Down syndrome with over 99% accuracy. Historically, this test was offered only to high-risk pregnancies (e.g. if mom was 35 years or older at deliver, having an abnormal screening test, etc.). However, newer guidelines suggest that amniocentesis be offered to all women, regardless of age. Amniocentesis can also look for evidence of open neural tube defects, such as spina bifida or anencephaly, if requested by your doctor.
Amniocentesis is a procedure done in your doctor’s office, and can rarely be done by an OB-GYN, but will often be done by a high-risk pregnancy doctor (a perinatologist working in a Maternal Fetal Medicine clinic). The doctor uses ultrasound to find where the baby is at and to locate a pocket of fluid that is furthest away from the baby. Continuing to use the ultrasound to guide, the doctor inserts a very thin needle into the abdomen until it enters the sac of fluid surrounding the baby. Because the uterus is a muscle, and the needle needs to pass through it to get to the amniotic sac, some women can experience cramping during this part of the procedure. A small amount of fluid is removed by a syringe, and then the needle is removed. The entire need part of the procedure generally takes a minute or less.
The fluid is then sent off to the laboratory where the cells from the fluid are grown, and then the lab can look at the chromosomes. The chromosomes are evaluated for any extra (trisomy, like Down syndrome) or missing chromosomes (monosomy, like Turner syndrome). Large deletions (missing sections of a chromosome) or duplications (extra sections of chromosome), and many different types of rearrangements can also be detected. The report the lab sends out has the results of all the chromosomes, including the sex chromosomes. So, if you don’t want to know if you’re having a boy or a girl, make sure to let your doctor know so they don’t spill the beans!
Besides looking at the chromosomes, the lab can also look for evidence of open neural tube defects (spina bifida or anencephaly) by measuring the chemical called alpha-feto protein (AFP) in the amniotic fluid (this is not automatically tested for with amniocentesis, but your doctor may request it). If the baby has an open neural tube defect (ONTD), the AFP will most likely be high. If it is high, the lab can go on to test the fluid for another chemical called acetylcholinesterase, which if found means there is a very high chance that the baby has an ONTD. ONTDs are a birth defect affecting approximately 1 to 2 of every 1000 babies born. Risk for a pregnancy affected with ONTDs does not increase with age. For more information on open neural tube defects click here.
Amniocentesis results with traditional chromosome analysis are typically considered to be very accurate when looking for chromosome conditions, such as Down syndrome. Results are also very accurate if you are having the procedure to look for other specific genetic conditions based on family history, such as cystic fibrosis or Tay-Sachs disease. However, no test is perfect and there are cases when the results are hard to interpret, or rare cases of lab error. With traditional chromosome analysis you will receive information on the chromosomes, gender, and possibly the risk for ONTDs, generally within about 10-14 days. Normal amniocentesis results also do not rule out all genetic conditions or birth defects.
Amniocentesis was historically offered to women age 35 or older to look for Down syndrome, as this was one of the first conditions known to increase as mom’s age increases. As time has passed and technology has improved, Amniocentesis with traditional chromosome analysis is now capable of detecting a variety of chromosome conditions, ranging from trisomy 13 and trisomy 18 to Turner syndrome. No matter what condition has been diagnosed, we know that this is not necessarily an easy time or what you expected.
If your results indicate a chromosome condition or birth defect, it is often very helpful to take time to talk to your provider, a genetic counselor, friends and family, and even get a second opinion. Taking time and consulting with others is especially important if your results are confusing, unclear, or if you don’t know much about the condition that has been diagnosed.
Even if you know a bit about the condition, it’s often helpful to get up-to-date information as things have changed dramatically. For example, in the case of individuals with Down syndrome, quality of life and life expectancy is quite different than it was two generations ago. Also, depending on what website you go to, who you talk to, and what you read, you are going to find differing opinions on what life may be like for a child with the condition that has been diagnosed, what it is like to parent a child with the condition, etc. Some people benefit from finding someone who has a child with the same diagnosis and speaking with them directly. Your provider or genetic counselor may be able to help facilitate this.
Ultimately, you have 3 options if you find out through amniocentesis that your baby has a chromosome abnormality, genetic condition, or birth defect….
Deciding whether or not to have a diagnostic test, such as amniocentesis, is a big decision. For some it may be a fairly easy call. For others it may be agonizing. Either way, it deserves thoughtful consideration.
Patients often ask their doctors or genetic counselors, “would you have the amniocentesis?” This is a great question. However, the answer is different for each individual and circumstance. One of the most important aspects of prenatal genetic counseling is that whatever the patient decides is based on their personality, beliefs and values, not the providers. Instead we offer you some questions to consider: