Amniocentesis

Authors:  Dr Monica Pahuja*
                        Dr Michael Bethune*

What are the prerequisites for having an amniocentesis done?

The following women can be referred for amniocentesis:

• over 36 years of age;
• past history of a child with an abnormality;
• those who have had an abnormality in their foetus or unborn baby identified on an ultrasound scan;
• personal or family history of a chromosomal abnormality or a partner with such a personal or family history;
• high risk as a result of first trimester screening (see nuchal translucency scan);
• anxiety for other reasons about the possibility of their foetus having a chromosomal abnormality.

What are the absolute contraindications for an amniocentesis?

There are no absolute contraindications for amniocentesis.

What are the relative contraindications for an amniocentesis?

Low risk pregnancy with risk of spontaneous abortion far outweighing the risk of a chromosomal abnormality.

Patients on heparin need to have this stopped before the test is performed.

What are the adverse effects of an amniocentesis?

There are three main risks which will be fully explained when the patient signs the consent form:

• Risk of miscarriage. About 1 in every 300 women who have amniocentesis will have a miscarriage.
• Risk of introducing infection into the amniotic sac (the fluid around the foetus) during sampling. This is very uncommon.
• Leakage of amniotic fluid occurs in less than 1 in every 100 women and generally settles.

While amniocentesis is able to identify Down syndrome and genetic or inherited disorders, it needs to be noted that there are some chromosomal disorders for which there are no tests currently available.

Are there alternative imaging tests, interventions or surgical procedures to an amniocentesis?

Nuchal translucency in the first trimester is an ultrasound scan when, combined with maternal age and maternal serum levels of pregnancy hormones, can give us a risk ratio of the likelihood of Downs syndrome, trisomy 18 and trisomy 13. This is a statistical evaluation only and cannot definitively tell us an answer. It is only 85% accurate.

Chorionic villous sampling is done slightly earlier than amniocentesis and can examine the foetal DNA, which does not occur with amniocentesis. However, the risk of miscarriage is about 1% which is 2 -3 times higher than amniocentesis (see Chorionic Villous Sampling).

*The author has no conflict of interest with this topic.

Page last modified on 29/3/2017.