Amniocentesis is a prenatal diagnostic procedure where fetal cells from the amniotic fluid are obtained and used to determine whether a fetus’ chromosomes are normal or whether there is a chromosome abnormality present. CVS (chorionic villous sampling) is usually preferred to investigate for specific inherited genetic disorders such as cystic fibrosis, muscular dystrophy, fragile X and haemophilia; specific gene testing can be done for specific biochemical disorders.
Amniocentesis is usually performed after the membrane has fused to the uterine wall, after 15-16 weeks.
Amniotic fluid is the fluid in the uterus that surrounds the fetus. It is mostly made up of the fetus’ urine. As with our skin cells, fetal skin cells are constantly being shed. The amniotic fluid is rich in fetal skin cells and fetal cells lining the lungs and the bladder.
During amniocentesis, a small amount (about 15-20 mls) of amniotic fluid (which contains these fetal cells) is removed and sent to a pathology department, where a specialised medical scientist, or cytogeneticist, will culture (grow the cells) and then examine them through a microscope. It is a very labour-intensive procedure that takes an average of 10-14 days to arrive at a final or long-term culture result. A special biochemical test such as an alpha-fetoprotein (AFP) may be done to help exclude neural tube defects such as spina bifida or anterior abdominal wall defects.
A more rapid chromosome test examining the five most common chromosomal abnormalities is sometimes but not always possible after 48 hours. This is called a “FISH” (fluorescent in-situ hybridization) test. This quicker FISH result is an additional test on the same specimen with an additional charge, but does not exclude all chromosomal abnormalities. Sometimes when a specific abnormality is suspected from abnormal ultrasound findings, it can be beneficial in expediting certain obstetric management decisions.
Common reasons to consider amniocentesis
Mother’s age is more than 35 years by the due date. Women over 35 years, based on their age risk, are at “high risk” of having a baby with a chromosomal abnormality. (The most common and best known is Down syndrome.)
A screening test such as the 12-week nuchal translucency scan, a 19 week morphology scan or an NIPT test has indicated a “high risk” result.
There has been a previous pregnancy affected by a chromosome abnormality or inherited genetic disorder.
There is a known inherited genetic disorder in the family of either the mother or father of the fetus.
Counselling prior to making the decision to have amniocentesis
The decision to have an amniocentesis is often a difficult and stressful one to make. It is important that the patient and her partner have fully discussed the issues prior to having the procedure done. They should discuss this amongst themselves and with their doctor. The decision to request amniocentesis should only be made when the couple feels fully informed and understand what is involved.
For this reason, SO+GI employs expert genetic counsellors. Pre-test genetic counselling will enable you to speak to a specifically trained genetic counsellor or radiologist prior to having the amniocentesis. Issues you can expect to discuss will include:
- What is actually involved in the test itself
- Which test is most suitable for you amniocentesis or CVS
- How long the results will take
- If the result is abnormal, what dilemmas you may feel
- Is testing the right thing for you?
The limitations of amniocentesis
Not all abnormalities can be diagnosed with amniocentesis. Only chromosome abnormalities and, when specifically tested for, certain genetic disorders can be diagnosed. Even if the chromosome result is normal, this does not guarantee a normal baby. It does, however, exclude major chromosomal abnormalities in the fetus.
Results of amniocentesis are very accurate, but occasionally the results can be very difficult to interpret. In such circumstances further chromosome testing may be required.
Amniocentesis is most commonly performed to determine whether or not the fetus has Down syndrome or another major chromosomal abnormality present. Occasionally, a rare chromosome abnormality may be found. This will usually best be explained to you by a geneticist who is a specialist doctor.
The accuracy of a specific test will depend on the specific inherited genetic disorder and will be discussed with by the genetic counsellor or your doctor.
In about 1 in 1,000 amniocentesis procedures, the fetal cells fail to grow. This is not due to a failure of the needle test or pathology laboratory. Rather, it is just a fact that some fetal cells fail to grow outside the body in the special laboratory environment. If this does occur, it does not mean that there is a problem with the fetus. Further testing may need to be performed.
Placental cells collected at CVS or fetal cells at amniocentesis may display “mosaicism.” This occurs when the fetus has two “cell lines,” meaning that one cell line contains normal cells in one culture and the other may contain abnormal cells in the other culture. If different cell lines are present in the amniocentesis sample, your amniocentesis result is inconclusive, and your doctor may order further testing.
The maternal cells may contaminate the fetal cells and give an incorrect or “maternal cell contamination” result. Laboratory staff are skilled to differentiate between adult and fetal cells, so this is rarely a problem.
Amniocentesis is always done after 15 weeks (when there is plenty of amniotic fluid present) to prevent a high percentage of “failed results” and club foot problems that have been seen when amniocentesis was performed before 14 weeks in the past.
An abnormal result
Most amniocentesis results will be normal. However, if the amniocentesis result is abnormal, the patient may require further information and counselling. SO+GI’s genetic counsellors are here to help you and are available for further counselling and will liaise directly with your doctor. If you have had prenatal testing performed at another practice, our genetic counsellors are still available for consultation.
Before the amniocentesis procedure begins, an ultrasound procedure is performed to the determine:
- Gestational age of the pregnancy
- Number of fetuses
- Position of uterus
- Placental location
- Fetal anatomy
- Fetal pathology
- Location of a suitable pocket of amniotic fluid to sample
The skin is cleaned with antiseptic solution and a sterile drape will be placed on the maternal abdomen. Under constant ultrasound guidance, local anaesthetic is placed into the skin. (You must tell the radiologist doing the amniocentesis if you are allergic to local anaesthetics or antiseptics before the procedure).
Again, under constant ultrasound guidance, an amniocentesis needle is then passed through the abdomen and the uterus into the amniotic fluid. The sharp stilette is then removed and a syringe is placed on the end of the needle. About 15-20 mls of fluid is then withdrawn into the syringe. The fetus quickly replaces this small amount of amniotic fluid.
Recovery from amniocentesis
During an amniocentesis, most women feel some discomfort. It is usually not a painful procedure. Often the stress and anxiety in the days before the amniocentesis are said to be the worst thing about the test by many patients.
After the amniocentesis, if the maternal blood group is negative, Anti-D is required within 72 hours. This will prevent the mother from developing antibodies to baby’s blood cells should the baby be blood group positive.
Patients who have had an amniocentesis are advised to take it easy for 24-48 hours. They are advised not to do any strenuous exercise or activity such as lifting or housework or picking up young children.
It is common for women who have had amniocentesis to experience some cramping for several days for which panadol may be taken (avoid aspirin as this can be associated with bleeding by thinning the blood)
FISH is usually available after two working days and is limited to looking at chromosome 21, 13, 18 and the sex chromosomes. Sometimes the lab will have technical difficulties and the FISH may fail. If this happens, it doesn’t mean that there is a problem with the baby. The lab will try and repeat the FISH but if this fails they will set up for the long-term culture.
Long-term culture takes approximately 10-14 days. Very rarely cells won’t grow and a result is not obtained.
CONTACT YOUR DOCTOR IMMEDIATELY IF YOU EXPERIENCE:
- Persistent or heavy bleeding
- Fluid loss from the vagina
- Severe abdominal pain or pain not relieved by panadol
- Signs of infection such as a fever or high temperature, sweats, chills, feeling unwell or a flu like illness.
Miscarriage will occur in 1:200 or 0.5% of amniocentesis performed. There usually is no specific reason for this. The procedure is relatively straightforward, although it does require proper training and experience and should only be done by experts who regularly perform amniocentesis.
The risk of miscarriage is brought about by the fact that a needle is being passed through the abdomen into the uterus. This is an unnatural thing to do and therefore carries the risk of causing the following that can lead to miscarriage:
- Introducing infection from the skin despite the sterile conditions
- Infection coming from the birth canal or from the blood stream
- Uterine contractions
- Leakage of some or all of the amniotic fluid through a hole in the membrane where the needle passed
If miscarriage does occur, it usually does so within two weeks of the amniocentesis.
There is a background risk of miscarriage in the first trimester in patients who have not undergone amniocentesis and it is not an infrequent occurrence and occurs in 12% of all recognised pregnancies.