Who may be offered amniocentesis?
1. Women with an abnormal nuchal translucency test or maternal blood triple test.
2. Women who have an abnormality on the mid pregnancy ultrasound at 18 – 20 weeks.
3. Women aged 35 and above.
4. Parents who are known to have a chromosomal abnormality themselves or who have a family history of a genetic disorder.
An amniocentesis can be performed at any stage in pregnancy after 15 weeks.
How is the test performed?
The mother will have an ultrasound examination, which will confirm the dates of her pregnancy and show the position of the baby and placenta.
Using a slender needle, which is guided by ultrasound, the doctor can locate a pocket of amniotic fluid separate from both the baby and placenta and withdraw a small amount of fluid (approximately 20-30mls). It only takes about thirty seconds to draw up the small quantity of amniotic fluid.
Normally the fluid is used to carry out two tests.
1. There is a quick result called a fast FISH (Flourescent In-Situ Hybridisation), which is a preliminary result only. If this is abnormal the final result is required to confirm this.
2. A test is carried out on the cells which are present in the fluid. Using special procedures in the laboratory, the chromosomes in these cells can be examined and in this way it is possible to find out if the baby has Down syndrome, or any of the other less common chromosome problems.
After the needle is removed and the baby is checked again, the mother may leave the room immediately to empty her bladder and dress.
Any discomfort felt is usually minor. Almost without exception, women experience far less discomfort from the test than they had expected.
The doctor who is looking after you and your pregnancy will tell you the results of the amniocentesis. In most cases, he/she will also be able to tell you the sex of the baby if you want to know.
There are a number of other tests which can be carried out on the fluid but these are only performed when the parents are known to be at risk of having a baby with a certain genetic condition.
A comprehensive assessment of the baby’s structures cannot be performed at 15 weeks. Your doctor may therefore arrange for another scan at around 18-20 weeks when details of the fetal development are more easily seen.
What complications can occur?
The risk of the test causing a miscarriage is low, not more than 1 in 200.
In about 0.5% of patients, there is some leakage of the amniotic fluid through the vagina in the 24-48 hours after the procedure. In almost all cases this stops on its own and does not cause any problems to the baby or to the pregnancy. The amniocentesis test does not appear to be associated with any other complications to the baby or to the pregnancy.
The test, like any other, may fail to give a result, either because no specimen is obtained or because the laboratory cannot produce a result. Both of these events are uncommon.
What about the results?
The fast FISH is available in around 2 days.
This gives a result on the 5 main chromosomes most commonly associated with abnormality.
The full chromosome testing takes around two to three weeks because of the requirement to grow the cells. The laboratory will send the results directly to your doctor who will arrange for you to receive the results