Amniocentesis with Traditional Chromosome Analysis

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.

How to Decide



The Basics

amniocentesis 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.

How it Works

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.

My amniocentesis results are abnormal: Things to consider

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….

  1. Continue the pregnancy and prepare: READ MORE
  2. Continue and place the baby for adoption: READ MORE
  3. Choose to not continue the pregnancy: READ MORE

How to Decide

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:

  • Do you feel like you need yes or no answers to be able to enjoy the rest of the pregnancy (i.e. eat, sleep, etc)?
  • Do you feel like you are OK waiting until the baby arrives to know for sure?
  • If you feel like you need more definitive answers, are you OK with the risk of miscarriage associated with these procedures (less than 1 in 200, or 0.5%)?
  • If you knew that the baby had a chromosome condition, would it affect your decision to continue the pregnancy?
  • Would you consider other options if there was a chromosome condition present in your baby, like placement of the baby for adoption or choosing not to continue the pregnancy?
  • Have you thought about these two scenarios: 1) you don’t have the amniocentesis and you find out at delivery that the baby has a chromosome condition, or 2) you do have the amniocentesis and you end up miscarrying a baby that did not have a chromosome condition. Both situations may be intense for some, but which scenario do you think you would be more OK with in the long haul? These are ethically challenging things to think and talk about. This is part of why your provider will allow you to make these decisions based on your own personality, beliefs, needs, and values.
  • Are there other trusted people in your life that you can turn to for advice and perspective (i.e. religious leader, family member, friend, etc.)?

Another way to look at this is: “What do other people decide to do?”

  1. Don’t get the amniocentesis because…“I wouldn’t do anything even if the test came back showing that the baby had a chromosome condition.”
  2. Get the amniocentesis because… “I wouldn’t do anything differently if the test came back showing that the baby had a chromosome condition, but I want to have the information to prepare.”
  3. Get the amniocentesis because…”I am not sure what I would do if the test came back showing a chromosome condition, but if it was for a serious disorder I may consider choosing not to continue the pregnancy.”

Amniocentesis FAQs

Questions? Get in touch


What are some reasons my doctor would offer me an amniocentesis?

Amniocentesis is now offered to women of all ages and some may choose to have amniocentesis simply because they want as much definitive information regarding age-related chromosome abnormalities as possible. Other reasons you may consider having amniocentesis include the following:

  • You had positive/abnormal results from a prenatal screening test, such as first trimester screening (FTS), sequential screening, quad screening, or cell-free DNA (cfDNA). In these situations, if you want yes or no answers it is recommended to consider having CVS or amniocentesis.
  • You have a family history of a chromosome abnormality, like a prior pregnancy. This puts you at higher risk and based on your previous experience, you may feel like you want to know what to expect during your current pregnancy.
  • You are 35 or older at delivery. Historically women 35 or older (advanced maternal age, or AMA) have been routinely offered amniocentesis because of the higher chance of chromosomal conditions, such as Down syndrome, as women get older.
  • You have a family history of a specific genetic condition, or you and/or your partner are known carriers of a genetic condition. If the genetic changes are known in your family, you may be able to have additional testing through amniocentesis to determine if your baby has a specific genetic condition, like Tay-Sachs disease, cystic fibrosis, spinal muscular atrophy, Fragile X, etc.

How can the procedure cause a miscarriage?

A common misconception is that the risk for miscarriage is there because of the potential that the doctor could poke the baby. Ultrasound imaging is so good that the baby rarely gets close to the needle. These days, the doctor can use a plastic cover for the needle, that way even if the baby were to brush up against it they would hit soft plastic rather than a sharp needle.

There are a couple of complications that can lead to a miscarriage. The first is if the amniotic sac tears. The vast majority of the time, after the fluid is removed, the needle comes out of the sac and the pressure from the fluid that is still inside the sac pushes the hole shut where the needle. In rare instances, the sac can rip or does not re-close the way it should and fluid can leak, causing a miscarriage.

Another possible complication is the same potential risk that comes with any invasive procedure: infection. Your doctor will take all necessary precautions (using fresh, sterilized tools, cleaning the area with antibacterial wash, etc.), but in some circumstances an infection can still happen no matter how many preventative steps we take. If an infection happens within the fluid around the baby, that can lead to a miscarriage.

Doctors that do these procedures generally have done them many times before. Newer studies have shown that miscarriage rates when the procedure is done by a doctor who is appropriately skilled are considerably lower than previously recognized.

Amniocentesis and CVS are both considered diagnostic, why choose amniocentesis?

Both amniocentesis and CVS are considered to be diagnostic tests to look for age-related chromosome abnormalities in the baby. There are a few advantages of amniocentesis:

  • Amniocentesis is considered less risky than CVS by some. However, in the hands of a specialist who regularly performs CVS, the risks are nearly equivalent.
  • Amniocentesis is looking directly at chromosomes from fetal skin and bladder cells, whereas CVS is looking at the chromosomes from placental cells (chorionic villi is part of the placenta). The placenta can sometimes have chromosomes that are different than in the baby.
  • In addition to looking for chromosome abnormalities, amniocentesis can also look for evidence of open neural tube defects (ONTDs), like spina bifida, while CVS does not. There are other ways to look at the chances of ONTDs besides amniocentesis, including level II ultrasound in the second trimester, and a blood draw done on mom in the second trimester to measure something called maternal serum alpha-feto protein (MSAFP). Open neural tube defects occur in approximately 1/500 to 1/1000 babies born and chances of this type of condition do not increase as mom ages.

Disadvantages of amniocentesis include the following:

  • Waiting until the second trimester. For various reasons, some people would like to know information regarding their baby’s chromosomes earlier in the pregnancy. CVS can be done in the first trimester as early as 10 weeks, whereas amniocentesis is typically done starting at 15 weeks gestation.

Genetic Support Foundation
Copyright © 2015 Genetic Support Foundation, Inc