This is a test which may be offered to a woman whose age, family or medical history, nuchal or blood test results suggest that she may have a higher than normal risk of having a baby with a chromosomal or genetic abnormality. Together you, your partner and your doctor can decide whether it is appropriate for you to have this test.
As the age of a mother increases, the chance of her having a baby with a chromosomal abnormality also increases. The most common chromosome abnormality is Down Syndrome. Between 35 and 40 years of age, the chance of having a baby with a chromosomal abnormality is approximately 1 in 150. Beyond the age of 40 years this chance increases even further.
This does not apply to spina bifida where the chance of having a baby with this condition does not increase with the increasing age of the mother.
While an amniocentesis can be performed at any stage in pregnancy, it is usually done at around 15 weeks. 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 very narrow needle which is guided by ultrasound, the doctor withdraws a small amount of amniotic fluid (about one tablespoon or 20ml) from the amniotic sac. It is not necessary to touch the baby at all.
Using special procedures in the laboratory, cells are grown in a culture medium. The cells are then fixed and stained. With high power microscopy the chromosomes in these cells can be examined and in this way it possible to find out if the baby has Down syndrome, or any of the other known less common chromosome problems. This is called a karyotype. Also, by looking at the chromosomes, the sex of the baby can be determined.
In order to get faster results, the laboratories have developed a technique called fast Fluorescent In-Situ Hybridization, where some of the cells from the sample are stained with a fluorescent dye. This can give us results for chromosomes 13, 18, 21, X & Y, in 24 hours. This is especially popular for women who have had a high risk result for Down syndrome (Trisomy 21)
Chromosome microarray (CMA) testing is a detailed genetic test which can detect extra or missing segments of DNA. As there are usually two copies of each chromosome, there should be two copies of each segment of DNA. Using the chorionic villus sample the microarray can look for a variation in the number of copies of the DNA. This is known as a copy number variant. If the test finds a copy number variant, the laboratory will check which genes are duplicated or missing. From this information we can predict what sort of problems the baby will have.
The amount of a particular protein alpha fetoprotein can also be measured. There is usually an increased amount of this protein present in the fluid when the baby has the abnormality called spina bifida. However these days, modern ultrasound equipment is better at detecting spina bifida than the amniotic fluid alpha fetoprotein test. So these days amniotic fluid AFP is only ordered under special circumstances.
It is not necessary for your bladder to be full. However, having a little urine in the bladder may make it easier to see the baby and the lower part of the uterus. We perform an ultrasound examination to check that the fetal number and size, and to locate the placenta and the biggest pool of amniotic fluid.
Then we sterilize the abdomen with antiseptic and create a sterile field with drapes. The needle is then inserted into the womb, guided all the time by the real-time ultrasound. It takes only about thirty seconds to draw up about a tablespoon of fluid into the syringe, then the needle is removed.
Fetal heart motion is checked again after the procedure. Then you are free to go to the bathroom and get dressed. We ask you to sit quietly for 10 minutes or so, to recover. Then you can go home and have a quiet day.
We ring you with the results as soon as they become available and we also forward them to your doctor. Fast FISH results are usually available in 24-48 hours.
The full karyotype takes 10-14 days. We ring you with both sets of results whether they are normal or not. If the results are not normal we will help you to decide what to do next, and who to contact.
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