An Amniocentesis is the removal of fluid (liquor) from the amniotic cavity. A fine needle is inserted through your abdomen into the womb under ultrasound guidance. Approximately 10 mls of fluid is required. The fluid from the amniotic cavity contains cells that have been shed from the babies skin. This is sent to a laboratory where these cells are extracted and cultured. It usually takes about 2 to 3 weeks for a sufficient number of cells to grow for an analysis of the chromosomes to be made. An amniocentesis is a relatively safe procedure in the hands of an experienced Obstetrician. Prior to having the procedure done, make sure you have a thorough understanding of the procedure and the significance of any results.

What Can we Learn from An Amniocentesis?

An Amniocentesis may be done for chromosomal analysis. This means that it will reveal if your baby has an abnormal number of chromosomes. Eg. Downs syndrome.
It does not help in the diagnosis of congenital abnormalities of the fetus except for open Neural tube defects (like spina bifida and anencephaly).
An uncommon use of amniocentesis is for the determination of fetal lung maturity (usually in Diabetic Mothers).
With Rhesus Isoimunisation an amniocentesis may be useful in determining the condition of baby.
Amniocentesis for checking for infection around the baby can be performed at any time during pregnancy.

Should I have an amniocentesis?

This will be discussed in depth with me.
The commonest reasons for performing an amniocentesis are maternal age or past history of a baby with a neural tube defect.
I generally discuss amniocentesis with all patients over 35, but the risks from amniocentesis (ie miscarriage) are theoretically higher than the rate of chromosomal abnormalities if you are under 37. This rate reverses after age 37.

Factors to take into consideration are

1. Whether or not you will terminate the pregnancy.
2. If not for termination, are you really willing to risk the pregnancy.

The decision to proceed must be entirely your own.

Are there any other tests available to determine genetic abnormalities?

1. The Triple test.
2. Nuchal fold thickness
3. Ultrasound scan for neural tube defects.

The above tests are available and have the advantage of being non invasive and carrying no risk to the fetus. The problems however still remain the false positive and false negative results.

They are just screening tests and a positive result should always be confirmed by a amniocentesis in the case of chromosomal abnormalities.

With neural tube defects however there are far fewer false results.

When is the procedure best done?

The procedure is best done at between 15 and 16 weeks. I do occasionally do the procedure earlier if requested but one should be aware that the earlier the procedure is performed the more the likelihood of not getting sufficient cells and the procedure having to be repeated.

Are there any Special Preparations?

You do not need to have starved for this procedure.
A full bladder is not necessary as for routine ultrasound examinations.
You should not perform any strenuous activity for 48 hours after the procedure.

Procedure for an Amnio.

1. Counselling conference done with me.
2. An ultrasound examination is performed.
3. Your partner or support person may remain with you throughout the procedure.
4. The needle will be inserted under ultrasound guidance.
5. The fluid will be withdrawn.
6. A further scan will be done to confirm fetal activity.

Common questions

1. Will local anaesthetic be used?
Local anaesthetic is not used as the needle for the anaesthetic usually causes more discomfort than the amniocentesis needle.

2. How much discomfort is there?
You will feel a menstrual-like cramp when the amnio is performed. It is rare to feel anything more than this.

3. Will my baby be hurt?
Injury to babies are rare. Continuous ultrasound monitoring is used. I do not place the needle near the baby, Very occasionally if the baby is very active I may defer the procedure. Once the needle is in place I have no control over babies movements. I usually use the area where babies legs are. If baby were to come into contact with the needle it would withdraw as we would if we came into contact with something sharp.

1. How soon is the fluid replaced?
Amniotic fluid is made by Baby passing urine. This is replaced within a few hours of withdrawal.


1. Miscarriage:
The miscarriage rate attributed theoretically to amniocentesis is 0.5 to 1 % (1 in 100 to 200). In general the more experience the clinician has, the lower the miscarriage rate will be. The miscarriage rate with me performing the procedure is much less than 1 in 200. Miscarriages happens spontaneously in 15- 20% of all New Zealand of all pregnancies, usually before the time that an amniocentesis would be performed.

2. Immediate risks:
Cramping may occur during amniocentesis and for a short while afterwards.
Bleeding and leaking of amniotic fluid occur only rarely and may mean a miscarriage is going to happen. You should contact me if any of this occurs.

3. Infection:
This is very rare. If this is going to happen, it usually occurs within 24 to 72 hours after the amniocentesis.

4. Repeat testing:
I may not be able to obtain amniotic fluid, or the laboratory analysis will fail to yield results. This does not mean that something is wrong with you or your baby. If results are desired, then amniocentesis must be performed again.

The following tests will be performed if you are having the amniocentesis for Genetic Reasons.

1. Chromosome analysis.
Fetal cells are extracted from the amniotic fluid and processed for chromosome studies. The most common chromosome abnormality is Down Syndrome. This test also determines the baby’s sex. You will need to let me know if you wish to know the sex of the baby.

2. Alpha-fetoprotein (AFP).
This test can help determine if there is an opening in the fetal skin. The most common place is the spine. This would be a neural tube defect, such as spina bifida or anencephaly.

What happens with the Results?
I will notify you when the results are available. I see all patients and give them the results in my office. NO results will be given out over the phone.

What happens if the test results are abnormal?
I will discuss in depth with you the abnormality, what it means, and the options available, including support services etc.
You will need to decide what to do about your own individual situation.

If the test results are normal, will my baby be healthy?
There is no test available that guarantees you will have a healthy baby. But, normal results help to exclude some potential problems and are reassuring. Only after your baby’s birth will you know just how healthy he or she is.


It is important for me to know your Rhesus status. If you are rhesus negative you will need to receive Anti D unless the babies father is Rh negative as well. I will discuss this with you at the time.
If you have any further questions, doubts, hesitations, please do not hesitate to discuss them with me prior to the procedure being performed.



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