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Frequently asked questions


What are some of the indications for prenatal screening?

  • Advanced maternal age (over 35 years)
  • Increased risk for a chromosome abnormality derived  by a first or second trimester biochemical test
  • Ultrasound findings (e.g. increased nuchal translucency at 1st trimester screen, cardiac abnormalities, intra-uterine growth retardation, cleft lip/palate etc.)
  • Family history of a chromosome abnormality such as a translocation
  • Previous pregnancy with a chromosomally abnormal outcome
  • Known family history of an inherited disorder such as cystic fibrosis, b-thalassaemia etc.

There is a family history of Down syndrome. What is my risk for having a child with Down syndrome? Approximately 95% of cases of trisomy 21 (Down syndrome) occur sporadically, i.e. they are not inherited. The most important factor determining the risk of having a baby with trisomy 21 is the mother’s age. Therefore, the risk for having a baby with trisomy 21 would be the same at the start of each pregnancy as any woman in the same age group. The risk is further refined following 1st trimester ultrasound scan and screening for biochemical markers.


A much smaller proportion of trisomy 21 babies (approx. 5%) is due to one of the parents carrying a chromosome rearrangement, known as a Robertsonian translocation between chromosomes 14 and 21. Carriers of a der(14;21) translocation have an increased risk for having offspring with Down syndrome. This translocation can be detected by examining the peripheral blood of both parents.


What information does the karyotype provide?
A karyotype looks at the number and structure of all 46 chromosomes in the cell. It can therefore provide information with regards to the gross morphology of the chromosomes and their number, as seen down the microscope. However, some chromosome rearrangements are too subtle to be seen down the microscope and need specialized molecular techniques for their detection, such as MLPA or CGH. A number of clinical syndromes such as Prader/Willi, Williams, Di George, etc. are due to microdeletions – small losses of DNA material -  that are not visible by conventional microscopy techniques. This is why we have introduced the Superscreen™ as a routine test in our laboratory.


If the QF-PCR result is normal, is everything OK? In 98-99% of cases, a normal QF-PCR result means that you have a very low risk for an abnormal baby. However, QF-PCR, cannot detect any chromosome abnormality for chromosomes other than 13, 18, 21, X and Y and it cannot detect numerical abnormalities for these chromosomes in low-grade mosaicism. Also, it cannot detect any structural abnormalities for any chromosome. This is why we also provide a karyotype and other forms of prenatal screening.


If you find a chromosome abnormality, does it mean that my baby will be affected? Not always – sometimes a chromosome abnormality, such as a translocation or inversion, has been inherited in exactly the same form from one of the parents. In this case, the risk for your baby to be abnormal is very, very low.


If we cannot find the abnormality in either parent (de novo), we strongly recommend the Superscreen extra™, to exlude any submicroscopic rearrangements. If the Superscreen extra™ is normal, the risk for your baby to be abnormal is very, very low.


In other situations, where we find an extra chromosome in mosaic or non-mosaic form, or other non-balanced abnormalities, we will assess your individual case. We will invite you to a counselling session to discuss the case with you and how it will affect your baby. We will also discuss the genetic risk for future pregnancies and for other members of your family.

 

What is mosaicism?
Sometimes we find a proportion of the cultured cells of the baby with the normal chromosome complement (46,XX and 46,XY) and a proportion with a numerical or structural abnormality. Thpresence of 2 or more different cell lines is known as mosaicism. Mosaicism does not always mean that your baby will be affected. Sometimes, abnormal cells are only found in the placenta and other extraembryonic tissues, but not in the embryo. In this case, your baby is most likely to be completely normal. If there is a risk that the abnormal cell line forms part of the embryo, we will provide genetic counselling and discuss the potential risks for the baby.


Why does it take so long to get the karyotype result?

Chromosomes are amenable for study only from a very specific part of the cell cycle known as metaphase. In order to obtain metaphases to study your baby’s chromosomes, we need to culture cells from your prenatal sample (amniotic fluid or chorionic villus). We culture them in special flasks in enriched culture medium under sterile conditions. This normally takes 10-12 days. We then harvest the cells and permanize them on glass slides before we analyse them down the microscope at 1,000x magnification.

What is a heritable disorder?  How is it inherited?
The majority of genetic disorders are inherited from one or both parents. Genetic disorders occur because of a defect in our DNA, in one or both copies of a particular gene (we all carry one paternal and one maternal copy for each one of our genes). A DNA defect in one copy of the gene, either paternal or maternal is rarely expressed physically, i.e. there are no symptoms of a disease, because the presence of a second, intact copy is usually sufficient to ensure a normal function for this gene.  Therefore, most genetic disorders are entirely harmless in an individual, known as the carrier, who carries one defective and one normal copy of the gene. However, if two defective genes are passed on by two parent-carriers, then they will cause the disease in the child. This form of genetic inheritance is known as recessive: the majority of genetic traits falls under this category, for example cystic fibrosis, a- and b- thalassaemia, spinal muscular atrophy and thousands of others.

Typical inheritance pattern of a recessive disorder, in this example cystic fibrosis.
A smaller proportion of genetic traits are dominant – this means that the inheritance of one defective gene from either parent suffices to cause the disease. Examples of these are Huntington’s chorea, dwarfism and others.
Finally another group of genetic traits are inherited through the sex chromosome X and, therefore genetic defects are unmasked at a higher frequency in boys (who only carry one copy of chromosome X), than girls (who carry two copies of chromosome X). These are known as X-linked disorders, and examples of these include colour-blindness, Duchenne muscular dystrophy, haemophilia and others.

 
   
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