Preimplantation Genetic Diagnosis (PGD) and Advanced Embryo Selection
PGD refers to genetic testing of embryos created by IVF, to diagnose embryos affected by genetic disease. Only embryos clear from the genetic disease in question are transferred to achieve a pregnancy.
PGD requires the biopsy of an embryo with removal of several cells for testing. To ensure the removal of cells does not impair the embryo’s potential to result in a pregnancy, the biopsy is usually conducted on day 5 of development (blastocyst) where the embryo has many cells. The area of the embryo that is sampled is the trophectoderm – destined to become the placenta. The inner cell mass, destined to become the baby itself is untouched. Sophisticated genetic diagnostic techniques, including Polymerase Chain Reaction, Karyomapping, massively parallel gene sequencing, comparative genomic hybridization and single nucleotide polymorphism arrays are used to determine both the chromosome makeup of the embryo and whether an inherited genetic mutation or chromosome rearrangement (translocation) is present.
PGD can help families at risk of having a baby with a serious or lethal genetic disease to have healthy babies.
Spontaneous chromosome abnormalities in embryos are increasingly common in women aged 35 and over. Advanced genetic technologies can be used to screen embryos for chromosomal abnormalities, ensuring only chromosomally normal (euploid) embryos are transferred. This is thought to increase a woman’s chance of pregnancy per embryo transfer, and reduce both her time-to-conception and her risk of miscarriage.
Preconception Genetic Screening
Genetic carrier screening refers to screening for the presence of “silent” (recessive) genes in two parents, that, if inherited together, could result in a baby suffering from a severe disease (e.g. cystic fibrosis). Dr Raelia Lew recommends that all couples consider undertaking genetic carrier screening pre-conception.
For most Australians, screening for three common inherited conditions is offered: cystic fibrosis (CF), fragile X syndrome (FXS) and spinal muscular atrophy (SMA). Many people are carriers of CF, FXS or SMA even though they do not know of anybody in their family who has the condition.
Where a patient’s genetic heritage increases their carrier risk, preconception screening for other specific recessive genetic conditions may be recommended. Examples include Ashkenazi Jewish, Middle Eastern, Mediterranean or South East Asian heritage due to increased prevalence of specific conditions in these groups (e.g. Tay Sachs disease, Thalassaemia).