An amniocentesis test is a maternal screening where a little of the amniotic fluid is extracted for testing. Amniocentesis is used to identify genetic disorders such as Down syndrome or other inherited disorders such muscular dystrophy or cystic fibrosis.
How does an amniocentesis test work?
The amniotic fluid that surrounds your unborn baby contains cells that have been shed by your baby. By extracting a little amniotic fluid, a sample of your baby's cells are also taken. These cells are grown in a lab until there are enough to map your baby's genes or chromosomes.
While amnio tests are considered to be up to 99% accurate not all birth defects can be detected with this test.
When is an amnio test done?
Usually an amniocentesis test is performed between 15 and 18 weeks of pregnancy. While it is possible to conduct tests earlier and later than this, amnio tests done earlier than 15 weeks are usually only done in extreme cases as they have higher rates of complications. Amnio tests done later than 18 weeks usually screen for other medical issues, such as lung maturity, rather than genetic and inherited disorders they are usually used to diagnose.
How is amniocentesis done?
An amniocentesis is performed by inserting a needle through your belly into your uterus to extract a little of the amniotic fluid that surrounds your baby. The needle is guided by an ultrasound to avoid contact with your baby or the placenta. It is not uncommon for your doctor to have to move the needle around to get the fluid needed. No anaesthetic is used during this procedure.
About 20mls of amniotic fluid is extracted for the test - this amount represents about 10% of your baby's total amniotic fluid at 15-16 weeks.
During amniocentesis, many women experience cramping or mild contractions. This is completely normal and may continue for a day or so after the test.
Doctors recommend that you completely rest for 24 hours after the procedure.
Amniocentesis test results: What do they mean?
The result of this test can take between two and three weeks to come back because of the time it takes to grow the cells and then do the gene analysis.
- An amnio test can tell you with 100% accuracy the sex of your baby.
- A normal result means that none of the genetic or inherited disorders the test looks for have been detected.
- Any test result that indicates your baby has a genetic disorder should be discussed with your health caregiver for clarification. They may refer you on to a genetic counsellor to discuss your options.
The risks of amniocentesis
As an invasive procedure, amnio tests do pose small risks for your unborn baby, but rarely any risk at all for you. Some of the risks include:
Amniocentesis has between a 0.5-1.0% rate of miscarriage. This represents 1 in 200 to 1 in 100 miscarriages. Miscarriages can occur as a result of infection developing in the uterus, waters breaking, contractions that don't stop, or excessive bleeding. If miscarriage is going to occur as the result of an amnio test, it is most likely to happen in the days or week after the test was carried out.
Injuring your baby
Directly injuring your baby during amniocentesis is exceedingly rare because an ultrasound is used to guide the needle, but occasionally babies are pricked by the needle - though this doesn't usually cause any long term injury.
Respiratory distress at birth
Research has shown that babies who have had amniocentesis have about a 1% increase in the risk of developing respiratory distress at birth. Respiratory distress can be treated but may require your baby to be in the intensive care nursery after birth until he stabilises.
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This article was written by Ella Walsh for Kidspot, New Zealand's best family health resource.