The issues surrounding amniocentesis are complex. This information addresses the more common concerns. If you have further questions, please feel free to ring, or ask our clinical staff when you attend for your appointment. If, after discussion, you elect not to proceed, there is no pressure to do so.
Who decides if I should have an amniocentesis?
You do. Your obstetrician or our staff can give you information to help you decide, but the final decision is yours.
Why should I consider having these test?
The most common reason is to determine whether the baby has Down syndrome. Other reasons include a suspicious scan, a previous baby with a problem or a family history of certain conditions eg cystic fibrosis.
The only fetal abnormality that increases with maternal age, is that the fetus could have the wrong number of chromosomes. If you have a normal amniocentesis, and a normal scan at 18-20 weeks, your chances of having a normal, healthy baby would be high.
Different people accept different levels and types of risks. Some say they couldn’t risk a miscarriage or undergo a termination no matter what. Others say they couldn’t lay the burden of a Down syndrome child on their other children, either during their childhood or after the parents die. Other people say they probably wouldn’t undergo a termination, but would like to know how to prepare themselves if necessary.
What is my risk for having a baby with Down Syndrome?
There are various ways of assessing the risk. The chart seen below is based on maternal age at conception, and looks at the risk of having a live-born Down syndrome baby. They also assume no previous or family history of problems like Down syndrome. The charts are quoted later as they also include other figures that need explaining.
What are the risks of having an amniocentesis?
Up until recently, it was thought that the miscarriage rate for CVS was about 1%, but a recent study has shown that when the CVS is performed in a tertiary centre, the chance of miscarriage is considerably less being 0.22% or about 1 in 500.
This study also found the rate of miscarriage for Amniocentesis was also much lower when performed at a tertiary centre, being 0.11% or about 1 in 1000 (Ann Tabor, World Conference in Fetal Medicine 2016).
Comparing your risk from the table with these risks may help you decide whether to have the procedure, although we understand that there is more to the decision than just numbers. However, if the risk for finding something bad is higher than the risk of doing something bad, you may feel more comfortable about having the procedure.
Rarely, other problems may occur, for example, the procedure or the laboratory testing may not be successful.
It is important to understand that amniocentesis looks for specific things such as Down syndrome. It does not rule out all abnormalities. It is important to have a morphology scan at 19-21 weeks to exclude many other problems.
Think of the chromosomes as building plans, and the baby’s development as the building process. Amniocentesis ensures that the plans are correct, and the 19-21 week scan is like the building inspector checking up part way through the building process.
Unfortunately, there is no test that can exclude mental retardation from other causes, including cerebral palsy.
When is the procedure done?
An amniocentesis is routinely performed between 14-16 weeks, although it can be done later for various reasons.
How is the procedure performed?
After consultation, a preliminary scan is made to assess the baby as far as possible and to check the dates. Dr. Carmody then assesses the best position for the needle to enter to obtain the amniotic fluid. This depends on the position of the baby and the placenta.
The mother’s abdomen is cleaned with antiseptic and a local anaesthetic is given. This will sting briefly and then go numb. Most ladies say that this is the worst part.
The main needle is carefully inserted whilst scanning to avoid the baby. The lining of the uterus cannot be numbed without potentially compromising the baby. Some discomfort will be felt as the main needle passes through this lining, although it doesn’t worry most women.
20 mls of fluid is withdrawn. This takes a minute or two due to the narrow gauge of the needle. The needle is withdrawn and the baby is then rescanned to ensure that no problems have occurred.The whole procedure involving needles usually takes around five minutes.
Why is 20 mls of fluid needed?
The fluid contains a small number of skin cells that have washed off the baby. These are what the laboratory actually cultures(grows) to check the chromosomes. If we take less than 10 mls, the laboratory may not have sufficient cells.
The amount of fluid varies with gestation from person to person, but there is usually around 350-400 mls at the time of the procedure. The baby makes up the difference in a few days. Most women say that the procedure was not as bad as they imagined.
What happens afterwards?
Most women are able to continue their normal activities. Extreme activities should be avoided for a day or two. Some women experience a period-type pain almost immediately after the procedure. This is not related to miscarriage. It is the muscle of the uterus reacting to the needle. It usually settles fairly quickly.
Why does Down Syndrome happen?
Almost all cells in the body contain 46 chromosomes. There are 23 pairs: 22 general pairs (numbered from longest to shortest), and 2 sex chromosomes. XX is female and XY is male. The pattern of a normal female is shown below:
Chromosome pattern of a normal female
The baby receives one of each chromosome from each parent. Just before ovulation, the egg divides and changes from having 23 pairs of chromosomes into having 23 single chromosomes. These, when joined by 23 from the sperm, combine to return the number of chromosomes to 46.
The father’s age is not a concern as sperm regenerate every 40 days or so. However, women are born with all of their eggs. As the eggs get older, the bonds between the pairs get stickier. When the egg divides, the pairs may not separate cleanly. This is how some babies finish up with one extra chromosome and why it is more likely to occur as women get older. The viability of the fetus depends on which pair is involved. Most of the unusual combinations are not viable and miscarry before 12 weeks.
Down syndrome is specifically an extra chromosome number 21, i.e. 3 x 21’s. The technical term is Trisomy 21, (try-so-me 21). This is derived from tri-, meaning three and –somy, meaning chromosome. Trisomy 21 accounts for around half of all babies with an unusual combination of chromosome that have survived to 13 weeks.
Although Down syndrome is more likely to occur as women get older, it can happen to younger women sporadically for reasons that we don’t understand. In fact, around 80% of Down syndrome babies are now born to younger women. This is partly because older women are offered amniocentesis and the option of termination, but mainly because younger women are having more babies than older women.
How accurate is the test for Down syndrome?
The amniocentesis results are virtually 100% accurate, as the laboratory are looking directly at the chromosomes. The most commonly reported cause for incorrect results is writing the wrong name on the test tube. Dr. Carmody will ask you to check this.
Does the Amnio or CVS result show anything else other than Down syndrome?
Due to improving technology we are now able to do more than look at whole chromosomes. We are able to look within the chromosomes to see if there are any small additions of genetic material or parts missing.
These are known as microadditions and microdeletions. To check this information we perfom a microarray. It can tell us if thefetus has various syndromes including DiGeorge syndrome, Angelman syndrome, Prader-Willi syndrome and Cri du Chat. It can also detect some cases of Autism.
There is no extra laboratory cost to you when we test using the microarray.
What about Spina Bifida?
This is where the neural canal is not protected by the spine and is one of a group of problems called neural tube defects.
While the amniotic fluid is in the laboratory, they measure a substance called Alpha Feto Protein , or AFP. If the level is normal, this would make Spina Bifida unlikely. An elevated level of AFP in the amniotic fluid would detect 97% of cases of Spina Bifida.
How long will the results take?
The AFP results are usually through in a few days. The final microarray results usually take around 10-14 days, depending on the gestation and how much the cells have to be cultured. We will not disclose the gender of the baby unless you ask.
The Fluorescent In-Situ Hybridization, or FISH test should be available within the first 24 hours (working days). This test looks at the more common chromosome problems covering chromosomes 13 (Patau Syndrome) and 18 (Edward's Syndrome), 21 ( Down syndrome) and the sex chromosomes. These together account for over 90% of any bad chromosome results. Occasionally, there may not be enough skin cells in the amniotic fluid to perform both the FISH and the Final test. In this case, the laboratory will skip the FISH test and proceed with the Final test which includes all chromosomes.
After 10 to 14 days we receive the final result. Until recently, this result just counted the number of chromosomes present, but as Microarray technology is now available, we will be ordering Microarrays on all patients who have an Amniocentesis (see Microarray information above). The only exception to this will be in the cases of translocations.
We will ring you with the results as they come in - the FISH result will be the following working day. A copy will be sent to your doctor. You will be contacted regardless of whether or not the FISH was successful. If a chromosome problem is detected, Dr Carmody will contact you with the result, and you will be given the opportunity to discuss the findings. You will be told what the abnormality is and how this will affect your baby. You will have the chance to discuss the results fully before making any decisions.
I’m not keen on having an amniocentesis or CVS. What are the alternatives?
Unfortunately, the only way to be 100% about things like Down syndrome is to have an amniocentesis or CVS.
If you wish, there is now a blood test the mother can have where fetal DNA can be detected in her bloodstream (NIPT). This is a very good test when assessing the risk for Down syndrome (more than 99%) but has limits when assessing other genetic problems.
At 11 - 14 weeks, we can perform a Nuchal Translucency screening test. If this assessment places you in a high risk for Trisomy 21 it would raise our level of suspicion and we would recommend an amniocentesis. However, not all babies with Down syndrome have an increased fluid collection. That is, some would not be detected. The detection rate in the literature varies somewhat, but it is generally thought to be 85 - 95%.
At 19 weeks, the normal morphology scan can be performed. This may detect ‘markers’ or indicators that would make us suspicious of an increased chance of Down syndrome and we would recommend an amniocentesis. However, some babies with Down syndrome look totally normal on scan. Again, the detection rate in the literature varies.
You may be prepared to accept the above tests, but they cannot guarantee that your baby does not have Down syndrome. Unfortunately, the bottom line is that the only YES/NO tests available for Down syndrome are amniocentesis or CVS.
The NIPT should always be preceded by an ultrasound to check fetal gestation and viability.
The testing is now processed in Australia and results are available in about ten days. This test screens for :
Trisomy 21 (Down Syndrome) - 99.5% accurate
Trisomy 18 (Edwards Syndrome) - 97% accurate
Trisomy 13 (Patau Syndrome) - 94% accurate
Sex chromosomes - 99% accurate
Triploidy - accuracy not documented
The accuracy of the test is reduced when the patients are:
- insulin dependent diabetics
- have high BMI
- are less than 10 weeks pregnant
Failure of the NIPT increases the chance of aneuploidy by a factor of 9, so patients whose NIPT result has failed twice should be counselled and may consider invasive testing. The failure rate of the NIPT is 8%.
It should be noted that the NIPT does not replace the Nuchal Translucency screening test at 12 to 13 weeks but may be donein conjunction with it.
The NIPT is a screening test to look at the chance of the fetus having Trisomy 13, 18, 21 and having problems with the sex chromosomes.
The Nuchal Translucency screen also checks fetal structure (early morphology scan) and also assesses the chance of the patient developing early onset pre-eclampsia, low birth weight (IUGR) and risk of premature delivery.
At 12 to 13 weeks, we can perform a special ultrasound check for Nuchal Translucency, or increased fluid. If this is found beyond a certain limit for a given gestation, it would raise our level of suspicion and we would recommend an amniocentesis. However, not all babies with Down syndrome have this increased fluid. That is, some would not be detected. The detection rate in the literature varies somewhat, but it is generally thought to be 85%-95%.
At 19 weeks, the normal ‘Morphology Scan’ can be performed. This may detect ‘markers’ or indicators that would make us suspicious, and we would recommend an amniocentesis. However, some babies with Down syndrome look totally normal on scan. Again, the detection rate in the literature varies.
You may be prepared to accept the above tests, but they cannot guarantee that your baby does not have Down syndrome. There are differing opinions as the effectiveness of combining the results of the different tests.
Unfortunately, the bottom line is that the only YES/NO tests available for Down syndrome are amniocentesis or CVS.
This chart is based on maternal age at conception and looks at the risk of having a live-born Down syndrome baby. The second column is the risk of having a live born baby with the wrong number of chromosomes, including Down syndrome.
The actual risk of having a baby with Down syndrome is greater at 12 weeks gestation than at full term, as fetuses with Down syndrome often miscarry naturally throughout the term of the pregnancy.