Amniocentesis

What is an Amniocentesis?

An amniocentesis is a prenatal diagnostic test in which a small volume of amniotic fluid (typically 12-15mls) is removed from the sac around the fetus to enable the testing of the fetal chromosomes, DNA or infection studies. The fetus sheds cells from the skin and urinary tract which then enter the amniotic sac and can be obtained from the amniotic fluid by amniocentesis. Amniocentesis, unlike CVS cannot be safely performed in the first trimester and is typically conducted at 16 weeks gestation.

Why would an amniocentesis be performed?

An amniocentesis is performed to assess the fetal chromosomes in pregnancies with an increased likelihood for chromosome problems (eg. increased probability at the screening First Trimester Screen, high probability following NIPT, previous affected pregnancies, parental chromosome problem).

In certain circumstances an amniocentesis may be indicated to assess a pregnancy at risk for congenital infection (eg. potential cytomegalovirus or toxoplasmosis infection).

How is an amniocentesis performed?

An amniocentesis is performed with the use of continuous ultrasound guidance to facilitate accurate placement of the needle into the amniotic sac without interaction with the fetus.

A preliminary ultrasound examination is conducted to assess the pregnancy and pelvic structures. Following the application of povidone iodine to sterilise the maternal skin, a 22 gauge needle is inserted through the abdominal wall. We do not routinely administer local anaesthetic for amniocentesis procedures. The ultrasound transducer rests on the maternal skin at all times to ensure the needle enters the amniotic sac at the safest location so as to avoid contact with the fetus. A 10 cc syringe is attached to the amniocentesis needle once it is correctly positioned within the amniotic sac and 12-15 mls of amniotic fluid is aspirated. The needle is within the amniotic sac for about 30-60 seconds. In general only a single needle insertion is required.

Documentation of the maternal blood group is required at the time of the amniocentesis. Women who are Rhesus negative will need to have an injection of anti-D following the amniocentesis (which will be administered by the medical staff at Women’s Imaging Services immediately following the amniocentesis).

When will the test results be available?

In general, a preliminary chromosome result is available in 24- 36 hours. The full fetal chromosome result will take 10-14 days on average.

normal male karyotype
Normal Male Karyotype

normal female karyotype
Normal Female Kayotype

These two pictures demonstrate a normal chromosome result for a boy (left) and girl (right). In a normal fetus there are 46 chromosomes in each cell of the body (22 pairs of autosomes and 2 sex determining chromosomes).

Are there any risks to having an Amniocentesis?

An amniocentesis is an invasive diagnostic procedure and does carry with it a small but definite pregnancy loss rate (in the order of 0.5-1.0%). The presence of uterine fibroids (benign tumours of the uterine muscle) or prior vaginal bleeding may increase the pregnancy loss rate following an amniocentesis.

Fetal injury following an amniocentesis is rare. It is common to feel some uterine cramps or discomfort following an amniocentesis. Vaginal bleeding or fluid loss following an amniocentesis is uncommon and requires prompt medical attention.

We strongly suggest you have some-one attend your amniocentesis appointment with you and have them drive you home. It is not appropriate to have your children attend for an amniocentesis procedure and we would request you arrange childcare when scheduling for this procedure.

When will the test results be available?

In general, a preliminary chromosome result (QF PCR or FISH) is available in 24-36 hours. The full fetal chromosome result will take 10 days on average to complete. The cytogenetic laboratory (Western Diagnostic Pathology or Path West) will issue a separate account for the chromosomal testing.

What are the advantages of an amniocentesis?

The main advantage of an amniocentesis test is that, unlike a CVS, it directly assesses the fetal karyotype. Issues such as confined placental mosaicism which may occur in 1-2% of cases of CVS do not occur.

As amniocentesis is performed later in pregnancy than CVS it has a lower pregnancy loss rate.

A preliminary chromosome result may be available in 24-36 hours.

Are there any disadvantages to an amniocentesis?

As discussed above, there is a pregnancy loss risk of 0.5-1% following an amniocentesis, even in experienced hands. The pregnancy loss rate increases with the number of attempts required to obtain the amniotic fluid, however in the majority of cases we require only a single needle pass.

Non-invasive prenatal testing

Non-invasive prenatal testing (NIPT) is now available to women in Australia who have a pregnancy at increased probability of trisomy 21 (Down Syndrome), trisomy 18, trisomy 13 and abnormalities of the sex chromsomes (eg. Turner's syndrome or Klinefelter's syndorme). NIPT is a blood test from which cell-free fetal DNA is extracted and can detect, with a high sensitivity and specificity, the chance of the baby having these specific chromosomal disorders. This form of testing carries no direct risk to the fetus and is proving very popular in Australia. Testing may be performed from any time after 10 weeks gestation, although it is strongly recommended an ultrasound is performed prior to testing to ensure there is no contraindication to testing (eg. incorrect dating, a major fetal structural problem). It is important to recognise that NIPT is a high-level screening test and therefore associated with false positive results (ie. reporting there is a problem when in fact there is not) and false negative results (ie. saying all is well when it is not). The chance of a false positive result increases in women who have NIPT when their pregnancy is at very low risk of a chromosome problem (ie a risk >1:2,000).

Women who may consider accessing NIPT include those with:

  • Increased probability of fetal chromosome problems following First Trimester Screening
  • Previously affected pregnancy with trisomy 21
  • Advanced maternal age

NIPT is regarded as a high-level screening test for some chromosome problems: if NIPT indicates the fetus has an increased probabilityfor one of these 3 trisomies, an amniocentesis is required to confirm the diagnosis.

The tests are now performed in Australia and it will take, on average, 5-8 working days for a result to be available. There are currently four providers of NIPT for women in Western Australia:

  • Sonic Genetics (Harmony): $450/test (Clinipath, multiple sites in WA)
  • Genomic Diagnostics (Generation): $395/test (Western Diagnostic Pathology, Myaree laboratory)
  •        Roche (Harmony): $525/test (PathWest, multiple laboratory sites in WA)
  •        Natera (Panorama): $695/test (SJOG Pathology, multiple sites in WA)

NIPT does not attract a Medicare benefit and you will be required to provide full payment to the pathology laboratory at the time of the blood draw. The price of NIPT charged by the service providers is altering on a regular basis (typically downwards) and the costs listed are an approximate estimate.

If you are considering using NIPT, the medical staff at Womens Imaging Services will be able to assist you in understanding the advantages and disadvantages of this form of testing. If you decide that NIPT is an appropriate testing modality for your pregnancy, the medical staff at Womens Imaging Services can facilitate the conduct of this testing for you. The staff at Womens Imaging Services have no contracts with any of the companies providing NIPT in Western Australia. A current position statement on Non-Invasive Prenatal Testing is available from the International Society for Prenatal Diagnosis.

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