Examination of amniotic fluid (amniocentesis)
Hardly any other examination in the field of prenatal screening has been distorted as much by exaggerations, half-truths and self-proclaimed experts as the examination of amniotic fluid (amniocentesis). On the internet, you can often find complication rates of 2% to 4%. Many expectant parents believe that one of the greatest risks of amniocentesis is the risk of injury to the foetus with the needle. However, objective and sound information on amniocentesis can answer many questions and also dispel the expectant parents’ fears.
Of course, no presentation on the internet whatsoever can ever replace a personal consultation, e.g. consultation within human genetic counselling.
Amniocentesis is usually performed between the 14th and the 19th week of pregnancy in order to detect chromosomal abnormalities. Foetal cells in the amniotic fluid are withdrawn and cultivated in a culture. After approx. 8 – 12 days, the chromosome set of the unborn child can usually be determined.
The chromosomal analysis is aimed at detecting chromosomal defects in terms of the number of chromosomes (the best-known defect is the Down syndrome = trisomy 21, previously known as mongolism) and structural chromosomal alterations visible under a light microscope. However, finer structural alterations can usually not be detected; further examinations with special markers will only be performed in case of suspected defects.
Moreover, the level of alpha-1-fetoprotein (AFP) in the amniotic fluid is measured to identify any neural tube defects of the spine (spina bifida = “split spine”) or the abdominal wall.
In special cases or upon request, an additional rapid test (FISH) can be offered to definitely rule out the three most common chromosomal defects after 24 hours already.
Amniocentesis may also be necessary for a diagnosis in case of other diseases (e.g. infections, inherited metabolic diseases, hereditary diseases known in the family history, blood type incompatibility).
A normal set of chromosomes does not exclude many malformations and diseases of the unborn child, e.g. heart defects, limb defects, facial clefts as well as many mental disabilities, because these are not linked to recognisable chromosomal abnormalities. Most of these diseases may, however, be recognised in a high-resolution ultrasound scan (the best time for such scan is between the 20th and 22nd week of pregnancy).
You will first have an extensive talk with your doctor, who will inform you about the performance, significance, limitations and risks of the intended examination.
After an extensive ultrasound scan of the unborn child, the position of the child and the size and expansion of the amniotic sac are assessed. Then the doctor determines the best puncture site which will usually be within the area displayed on the picture below.
Then, after thorough disinfection (three times) of the abdominal wall and under continuous ultrasound guidance, a very thin hollow needle will be inserted through your abdominal wall and into the amniotic sac in a way that excludes any injury to the foetus. A small amount of amniotic fluid (approx. 10-15 ml, less than a tenth of the total amount) will be extracted with the needle. This procedure usually takes about 20 seconds. The discomfort caused by amniocentesis can be compared to that of taking blood samples or intramuscular injections. Local anaesthesia is not necessary. The amniotic sac will have replenished the liquid within a few hours.
Behaviour after the puncture
After the puncture you should rest and lie down at home for the rest of the day; and for the following three days you should also stay at home and take it easy.