Amniocentesis

What is it?
There are many tests throughout pregnancy. However, amniocentesis is recommended for women over 35 and for those who fall into a high-risk category (a history of Down’s Sydrome in the family or much older mothers).

When would I need amniocentesis?
If you have an ultrasound scan or a blood test that suggests you are at high risk of having a baby with Down’s Syndrome or some other problem, you will be asked whether you want to have more tests in order to confirm the results.

Amniocentesis is considered for:
1) older maternal age
2) abnormal triple screen blood tests
3) abnormal findings on ultrasound
4) a family history of neural tube defects
5) family history of another genetic condition

What can amniocentesis show?
Many genetic conditions may be detected by DNA or biochemical screening (i.e. Tay Sachs, cystic fibrosis). Genetic diseases such as cystic fibrosis, Duchenne muscular dystrophy and sickle cell are detectable through prenatal testing. With the rapid advances in discovery and technology, this list is quickly expanding. Amniocentesis is generally recommended when there is a possibility of Down’s Syndrome. Take heart in the fact that only one baby in 700 in Hong Kong has Down’s Syndrome, which is low.

How is amniocentesis performed?
Amniocentesis is the procedure of withdrawing a small portion of amniotic fluid that is surrounding the foetus. Standard genetic amniocentesis is usually performed between 15 and 18 weeks and is used in the prenatal diagnosis of certain genetic diseases. Amniocentesis is performed in conjunction with an ultrasound scan that determines the size and position of the foetus. The ultrasound can also monitor foetal development. When the position of the foetus is determined, a needle is placed into the abdomen and through the uterus allowing the withdrawal of approximately 2 tablespoons of fluid. Most women find that this procedure is not painful and describe it more as a feeling of pressure.

How safe is the procedure?
The safety of amniocentesis is considered to be approximately 99.5%, but as with any invasive medical procedure there is some degree of risk. The procedure rarely leads to miscarriage or fetal loss. The increased risk for complications including bleeding, heavy cramping, amniotic fluid leakage, infection or miscarriage may be up to 1/2%. There is only a one in a 100 risk of amniocentesis causing a miscarriage.

Is genetic evaluation necessary?
A genetic evaluation (genetic counsellor, geneticist or OB/Gyn) is recommended prior to having an amniocentesis. A family history is taken at this consultation and based on that information, further genetic testing may be offered. The risks to the pregnancy and the availability of carrier and prenatal testing will be discussed with you. You will be asked about exposures to medications, chemical and alcohol during the pregnancy. The genetic counsellor will help you clarify your concerns and provide the information necessary to facilitate your decision-making. Prenatal diagnosis and the decisions that follow an abnormal finding in the pregnancy are very personal ones and only you know what is best for you and your family.

What could be discovered through amniocentesis?
The diagnoses included in standard genetic amniocentesis include chromosomal analysis and alpha-fetoprotein (AFP) measurement. Chromosome analysis of the amniotic fluid examines the sample for extra (Down's syndrome), missing (Turner syndrome) or abnormal (translocations) chromosomes. Most fetal chromosome anomalies are not inherited but occur more frequently as women get older. A standard chromosome analysis does not rule out missing genes or even tiny pieces of missing chromosome material. There are over 50,000 genes, far too many to examine in a pregnancy. The accuracy of the chromosome analysis through amniocentesis is well established and extremely high (greater than 99%). Amniotic fluid AFP detects approximately 95% of severe neural tube defects (spina bifida and anencephaly). For families with a history of a known genetic disorder, prenatal diagnosis is increasingly available through studies on the amniotic fluid cells.

It is important to know that no prenatal test can guarantee a healthy baby. There is a risk of approximately 3-4% for any congenital birth defect in any pregnancy regardless of maternal age or family history.

When will I know the results?
The results from an amniocentesis are usually available within two weeks, depending on fetal cell growth. Usually the results are clear and unequivocal. However, occasionally further testing is necessary to help with interpretation. In the event that an abnormal result is found, the genetic counsellor, geneticist and obstetrician are available to assist you in understanding these results and the options available to you. Each family is asked to consider these aspects before deciding to undertake prenatal diagnosis by amniocentesis.

Being tested
If you are going to have a diagnostic test such as chorionic villus sampling or amniocentesis, or you are perhaps travelling to a major hospital to have a high-resolution ultrasound scan, try to take someone with you to support you. This is equally important if you are having a simple blood test but you have a needle phobia or are simply very anxious. Most people find antenatal tests nerve-wracking. While you are being tested:

Ask as many questions as you need to.

Try relaxing and realising that the risk of miscarriage is very small.

When you leave be sure to ask when results will be available and how they will be communicated to you.

It is nerve-wracking waiting for results. During this time talk about it to your spouse and friends and try to be positive.

Getting the news
Many people feel that their lives are on hold while they wait for the results of their tests. If, when the news comes, it is good, you will probably feel that a great weight has been taken off your shoulders. Be sure to let your doctor know that you are anxious for results as soon as they are available. If the news is bad, your GP or a member of the hospital staff should speak to you personally and arrange to see you the same day or, at the latest, the next day. You are likely to feel distraught. It is difficult to think clearly:

It helps to go with your spouse to the appointment. If you feel pressured about making a decision (to perhaps terminate your pregnancy) you might want to talk to a counsellor.

The Antenatal Results and Choices (ARC) organisation (formerly called SAFTA) can discuss all your options with you if you find out that your baby has a serious health problem. No pressure will be put on you; you’ll be given time to think and time to ask all your questions. If you decide to have a termination, ARC will talk to you about the various ways in which this can be carried out.

Could the results be faulty?
A number of recent instances have come to our attention in which children with very small chromosome abnormalities (usually micro-deletions) have been born after routine amniotic fluid chromosome analysis has been passed as normal. This is a very uncommon phenomenon, but each such event obviously causes distress to parents, medical attendants, and the laboratories and genetics departments concerned. The majority of amniotic fluid samples sent for chromosome analysis, excepting those where there is prior reason to suspect some particular abnormality, is taken with a view to screening for common numerical chromosomal abnormalities, particularly Down syndrome. For these purposes, a chromosome count is established, and the chromosomes are also examined for gross structural abnormalities. For both technical and resourcing reasons, the quality of structural analysis on such samples is often conducted at a lower level than that which is required to reliably detect small and unexpected chromosomal deletions and other rearrangements. Although many structural chromosome abnormalities will be detected, those that fall below the limits of resolution of the analysis will frequently be missed.

It is clear that laboratories in this field must always operate to a satisfactory technical standard, and the Association of Clinical Cytogeneticists has encouraged this by its long-term sponsorship of an external quality assessment scheme. The purpose of this statement is to point out that even laboratories operating at or above these required standards will inevitably on occasion fail to detect a small rearrangement in a situation where there has been no prior suspicion directing attention to that particular chromosome region.

These cases can be regarded as part of the substantial proportion of causes of congenital malformation, such as single gene abnormalities and non-genetic conditions, which are not detectable by routine chromosome analysis. This should be made clear to all professionals, such as obstetricians, midwives, and general practitioners, who are engaged in discussing prenatal chromosome screening with women, and should form part of the background counseling prior to such prenatal screening.

In the unfortunate case of the birth of an abnormal child, such counselling will help reduce the possibility of the parents' inevitable distress being aggravated by anger, in the mistaken belief that a diagnostic error has occurred.