Amniocentesis

What is it?

An amniocentesis is a diagnostic test that can be done from the 15th week of pregnancy onwards. The advantage of amniocentesis over screening tests such as maternal serum screening is that it gives completely accurate information about fetal chromosomes before babies are born. Normal results from an amniocentesis test will therefore confidently exclude fetal chromosome abnormalities such as Down syndrome which is the most common known cause of major mental handicap.

Who should have an amniocentesis?

The decision to have an amniocentesis rests entirely with the individual patient. The most common reason for requesting this test is to reassure mothers who are in the age group of 35 years or older. The risk of Down syndrome is considered high when women having babies are in this advanced age group.

Amniocentesis is considered when a fetal abnormality is detected on an ultrasound examination and also when screening tests with maternal serum or ultrasound produce a high risk result for chromosomal abnormality such as Down syndrome.

Amniocentesis can occasionally be used for testing of genetic syndromes such as cystic fibrosis however this application is specifically for those parents with a known family history risk. This usually requires prior genetic counseling and testing of the parents as potential carriers.

How is the test performed?

This test is highly specialised and is best performed by an obstetrician gynaecologist sonologist whose primary area of practice is obstetrical and gynaecological ultrasound. Under ultrasound guidance to ensure the safety of the pregnancy, an extremely fine needle which is thinner than a blood test needle, is passed into the amniotic fluid sac in order to obtain a small amount of this fluid. The cytogenetics laboratory has special techniques for extracting and growing fetal cells from the amniotic fluid in order for the fetal chromosomes to be examined.

Is it safe?

There is a small risk of miscarriage generally quoted as 1 in 200. This figure is low considering the background risk of miscarriage that exists for every pregnancy. Amniocentesis is therefore considered a safe test and there is no significant risk to the mother from the procedure. As mentioned above this highly specialised procedure is best performed by experts in this field and Dr. Shawn Choong at Northern Ultrasound for Women has extensive experience in prenatal fetal procedures including routine diagnostic testing with amniocentesis.

How to best prepare for an amniocentesis?

Please bring evidence of your blood group. No special preparation is necessary beyond that for a basic ultrasound scan which means attending with a comfortably full bladder. The following is also advisable:

• Bring someone with you who can drive you home afterwards.
• No heavy lifting for 24 hours after the procedure.

How long do the results take to come back?

The full results on all the detail of the fetal chromosomes takes 12 – 14 days usually. This is because the cytogenetics laboratory require this time for the cells to divide and be examined microscopically. Your doctor will receive these results as soon as they are available. An additional test producing rapid results is occasionally advised for extremely high risk cases particularly if the gestation is advanced. This is only 99% accurate and does not however cover the full detail of all the fetal chromosomes therefore the standard amniocentesis testing as above is still carried out.