Prenatal Testing

Possible types of tests. Risks. Advantages and Disadvantages.

  1. Questions and issues in prenatal testing
  2. The Debate - Who should be able to have PGD and choose their baby?
  3. Pre-conception testing
  4. Pre-implantation diagnosis
  5. Amniocentesis
  6. Genetic screening
  7. Genetically modified babies

Test procedures

Pre-conception testing

A maturing egg produces two small cells referred to as polar bodies. These cells, containing a copy of the mothers chromosomes die after fertilisation. The chromosomes in these cells may give indications of the presence of aneuploidy (extra or missing chromosomes) if such is coming from the egg.

Risks: There are no risks for the mother. It does not affect the viability of the egg. It has a reduced chance of detecting genetic abnormality and would only be used if a syndrome was expected to be transferred via the mothers genetic material.

Pre-implantation diagnosis

Is done on the devloping embryo. A single cell is removed after the embryo (a blastula) has reached the 8 cell stage. and can undergo a variety of tests.

"Fluorescence in-situ hybridisation" or FISH uses fluorescent dyes to show and identify chromosomes 13, 18, 21, X or Y.  Each chromosome picks up a separate colour, can therefore identify the presence of extra or missing chromosomes.

All but one cases of missing chromosomes cause the embryo to naturally abort or not to implant.  The exception is the missing X chromosome which caused Turner's syndrome.

The most common syndrome from an extra chromosome is Down's syndrome which is caused by having an extra chromosome 21. Others are Klinefelter's, Patau's, and Edward's.

Amniocentesis

This involves the removal of small amount of the fluid surrounding a foetus while it is in the womb. The fluid is extracted by needle / syringe using ultrasound for guidance. The fluid is analysed for chromosomal abnormalities (e.g. Down's syndrome)   and other conditions e.g. spina bifida. The results can take several weeks to come through and can result in decisions about termination occurring late in pregnancy.

The risk of miscarriage is relatively low after the first three months. It is influenced by the skill of the person performing the procedure.

Genetic Screening

All the tests applied during pre-implantation diagnosis could be applied to material acquired during amniocentesis. Although generally speaking it would be only useful to conduct tests that gave meaningful information. If there was no indication of a possible problem, for example from family history or because of age,  it may be better not to test at all.