After cleansing the skin of the maternal abdomen with an antiseptic solution, this involves guiding a fine needle under ultrasound guidance into the fluid that surrounds the fetus . A small sample of the fluid is then taken off into a syringe and sent for testing.
What are the risks of amniocentesis?
The main risk is from miscarrige which occurs in about 1% pregnancies following amniocentesis but there is huge variation in miscarriage rates between different operators and Dr Beatties personal rate is around 1 in 300 (1/3%). There is also a risk of abdominal cramps and vaginal bleeding but if mild this does not usually cause any problems and you can take paracetemol safely for the cramps. If there is heavy bleeding, severe pain, fever, vaginal discharge or vaginal fluid loss you should contact your midwife, hospital, GP or our centre.
Why do you need to know my blood group?
Women who are Rhesus Negative need an injection of anti-D within 72 hours to protect their baby against the development of antibodies in the mother which can cause anaemia and jaundice in the fetus.
When is amniocentesis done and who will do the test?
Amniocentesis is performed after 15 weeks. All amniocentesis tests are carried out in our centre by Dr Bryan Beattie, Consultant in Fetal Medicine, who has perfomed over a thousand amniocentesis procedures in Cardiff.
What tests are done on the fluid?
Three main chromosome tests applied to the fluid are a PCR, a conventional karyotype and an array CGH.
1) The PCR works in 98% cases to give a result within 3 days for Down Syndrome (Trisomy 21), Edwards Syndrome (Trisomy 18), Pataus Syndrome (Trisomy 13) and if requested the sex of the baby.
2) A karyotype takes about 14 days and will identify most of the rarer chromosomal problems not picked up by the PCR.
3) Array CGH will identify up to 10% more chromosomal abnormalities that would not be picked up by a karyotype. This is the most accurate test and ideally we would advise this in all cases.
We also test for the dF508 cystic fibrosis genes which are responsible for about 85% of cases of cystic fibrosis.
Sometimes the amniocentesis may fail to give a result, give a result which is difficult to interpret or an unexpected result and if so it will be explained to you what implications this may have for the baby.
What do I do before and after the test?
The procedure requires no special preparation and takes about 20 minutes. Ideally try to arrange for someone else to drive you home. After resting for about 5-10 minutes you can go home to rest for at least 24 hours. You should book time off work the next day and try to arrange for help if you have small children to look after.
How will I get the results?
This will be discussed at the time of the amniocentesis but will usually be by phone followed by a report posted to you, the hospital and your GP.