An Amniocentesis is a diagnostic test performed after 16 weeks to assess the chromosomes of the fetus. It involves taking a sample of amniotic fluid using a needle passed through the mother’s abdominal wall under ultrasound vision and into the fluid around the baby.
Who may be offered an amniocentesis?
- Women who have had a screening test suggesting the fetus has an increased risk of chromosomal abnormality
- Women who have previously had a child with chromosomal abnormality
- Women who may be low risk for abnormality but want to definitively exclude chromosomal abnormality in their baby.
- Women in whom a fetal abnormality has been identified on ultrasound which may be associated with chromosomal abnormality
- Women in whom a fetal infection is suspected.
How is amniocentesis performed?
An ultrasound is performed to check the position of the placenta, the fetus and the amount of fluid around the baby.
The mother’s abdomen is then cleaned with antiseptic.
The very fine amniocentesis needle is then passed under ultrasound guidance through the mother’s abdominal wall and into the fluid around the baby. A small sample of amniotic fluid is then withdrawn through the needle.
What are the side effects?
Most women report the experience of the amniocentesis was better than they anticipated and was mildly uncomfortable rather than painful.
The sensation of the passing of the needle is similar to having a blood test taken.
There may be mild “period-type” pain as the needle passes through the uterine wall and then a “pinch” type sensation as it is withdrawn at the end.
A sensation of slight bruising or tenderness may continue for a day or two after the procedure.
What are the risks?
Amniocentesis is regarded as a safe procedure It confers a small increased risk of miscarriage to a patient’s pre-existing background risk. This is in the order of 1:500 or 0.2%.
Fluid leakage is a rare complication of amniocentesis. It occurs following approximately 0.5% of procedures. In most women it will stop spontaneously with no further complication. In a small proportion it may progress to miscarriage.
Technical failure at the time of testing or in the laboratory is a rare complication. Repeat amniocentesis would be offered in this instance.
What happens following the test?
If a woman is Rhesus negative, Anti D will be administered immediately following the procedure.
Patient’s are able to drive themselves home following the test but may prefer to have a companion do this.
It is recommended that heavy lifting, exercise and sexual intercourse be avoided for 24 hours following the procedure.
The tissue sample is sent to a specialized genetics laboratory where the placental tissue is grown in an incubator for 10-14 days before the cells can be analysed.
A limited result (known as the “FISH” test) is available 24-48 hours after the procedure at extra cost. This looks for an extra 13th, 18th and 21st chromosome and for the sex chromosomes
The full result of all the fetal chromosomes is known as a “karyotype” and is available 10-14 days after the procedure. Traditionally this has been a “banded karyotype” but new technology has allowed greater detail to be examined and this is known as a “microarray karyotype”. This has been shown to identify approximately 5% more chromosomal abnormalities than the banded karyotype but at the risk of 1-2% of a finding of uncertain significance. Parental blood tests are usually then required to clarify the result and counseling by a geneticist would be arranged to determine if the finding is significant in the individual case
The doctor performing the procedure will discuss the most appropriate test with you.
Additional information is available on The Victorian Clinical Genetic Services (VCGS) website here