Amniocentesis and CVS: Microarray analysis. Information for Parents
Microarray analysis
Microarray analysis uses modern methods to detect chromosome imbalances too small to see by previous chromosome tests. Changes in the number or structure of chromosomes can be associated with physical and intellectual disability in children. Microarray analysis is also called array Comparative Genome Hybridisation (CGH).
Why has microarray analysis been offered to me?
Tests that you have had so far in your pregnancy indicate that there may be an increased risk of a chromosome abnormality in your baby. Microarray analysis can be used to find a chromosome imbalance in the baby and may help to explain the ultrasound findings.
What are chromosomes?
Chromosomes are the structures found in the centre of each cell in the body and they contain our genetic information or genes. Genes are instructions which tell the body how to develop and function. Each cell has 46 chromosomes in 23 pairs. We inherit one of each chromosome from each parent to make a pair. The pairs are numbered from 1 to 22 from the largest to the smallest. The 23rd pair are the sex chromosomes X and Y. Girls usually have two X chromosomes (46, XX) and boys one X and one Y chromosome (46, XY).
Testing chromosomes
In the past chromosomes have been looked at with a microscope to detect visible changes in them and this picture of the chromosomes is called a karyotype. It can identify major chromosome problems but is not able to detect smaller changes because they cannot be seen with a microscope.
What are the advantages of microarray analysis?
▪ Microarray analysis can detect more chromosome imbalances than previous chromosome tests including some that are small in size but very significant. When small pieces of chromosome are missing these are called micro-deletions and extra pieces are called micro-duplications.
▪ An imbalance in the chromosomes may help to account for ultrasound findings and allow more precise information to be given about the implications. If you have this test, we will ask for a blood sample from both parents (when possible) to help interpret the result.
What are the limitations of microarray analysis?
▪ Microarray analysis will not detect very small chromosome imbalances or abnormalities in single genes. Many genetic conditions are not caused by chromosome imbalances but result from very tiny changes in individual genes and these are not covered by this test.
▪ Microarray analysis may detect chromosome changes of uncertain significance. This means that either there is not enough information at present to know if the change is important, or their effects are highly variable and therefore cannot be used to reliably predict the outcome for the baby. For this reason, genetic laboratories in the UK have developed a policy to report only on imbalances for which there is clear information.
▪ On rare occasions microarray analysis will identify an imbalance in the chromosomes which is not related to the ultrasound findings but could have implications for the future health of your baby or other family members. If this arises you will be offered an appointment with the Clinical Genetics team to discuss the result.
How will I be given the result?
The result will usually be available about 2-3 weeks after the procedure and a Specialist Midwife in the Fetal Medicine Unit will contact you. If any chromosome imbalances are identified you will be offered an appointment with the Clinical Genetics Team to discuss the result.
Further questions
If you have further questions about this test, please contact the Specialist Midwife in the Fetal Medicine Unit on 0117 342 5470
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