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 15-16 weeks gestation.
Why would an amniocentesis be performed?
An amniocentesis is performed to assess the fetal chromosomes in pregnancies at risk for chromosome problems (eg. increased risk at the screening First Trimester Screen, 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 for high risk cases. The full fetal chromosome result will take 10-14 days on average.
Normal Male Karyotype
Normal Female Kayotype
These two pictures demonstrate a normal chromosome result for a boy (upper) and girl (lower). 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 is available in 24- 36 hours for high risk cases. The full fetal chromosome result will take 10-14 days on average.
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 in very high risk cases.
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.
