Pre-implantation genetic diagnosis (PGD) or embryo biopsy
This picture, taken through a light microscope at high magnification shows a 4-cell embryo undergoing PGD, in which a single cell is being removed. The large suction pipette on the left holds the embryo steady whilst the smaller sharper pipette on the right pierces the outer protein covering of the embryo (the zona pellucida) and sucks out one of the cells.
The cell contains
all the genes of the embryo and short gene sequences from it are then positioned onto tiny microarrays ("biochips") for screening. A single
DNA sample can be screened for tens of thousands of gene sequences.
PGD involves the production of an embryo outside the body, by in vitro fertilisation (IVF), followed by removal and subsequent testing to destruction of one of the cells. IVF is a very invasive process, particularly for the woman, and the odds of a healthy baby being born after a single IVF cycle are quite low. At present there is little evidence to suggest that removing one of the cells of a four-cell embryo effects subsequent development.
Since its beginning in the mid 1990s there have been about 200-250 cycles of IVF where PGD is involved in the UK, and about 1300 worldwide, although this number is increasing each year. PGD pregnancy rates can range from 17-40% per cycle depending on the genetic condition.
By comparison over 18,000 pre-natal tests are performed each year in the UK. This incudes amniocentesis where a sample of amniotic fluid is withdrawn from inside the membranes surrounding the developing foetus at about 16 weeks of gestation. The baby's cells in the amniotic fluid are tested for chromosomal disorders such as Down's syndrome. By this time, the woman is in the second trimester of her pregnancy, and may have to face the distressing choice as to whether to terminate the pregnancy.
The Ethical Issues