Preimplantation genetic testing is a technique which involves checking the genes or chromosomes of the embryos , created through IVF or ICSI, before they are transferred to the uterus.
This technique helps to ensure that only healthy embryos are put in the uterus during treatment and provides an alternative to current postconception diagnostic procedures (ie.amniocentesis or chorionic villus sampling),which are frequently followed by the difficult decision of pregnancy termination if results are unfavorable.
The embryologist can perform a polar body biopsy, a blastomere biopsy or a trophoblast biopsy to obtain genetic information from the embryo.
Preimplantation genetic diagnosis (PGD)
This term applies when one or both genetic parents carry a gene mutation or a balanced chromosomal rearrangement and testing is performed to determine whether that specific mutation or an unbalanced chromosome has been transmitted to the embryo.
PGD should be offered for 3 major groups of disease:
1) sex-linked disorders
2) single gene defects and
3) chromosomal disorders.
X-linked diseases are passed to the child through a mother who is carrier. They are passed by an abnormal X chromosome and manifest in sons, who do not inherit the normal X chromosome from the father. Because the X chromosome is transmitted to offspring/embryos through the mother, affected fathers have sons who are not affected, but their daughters have a 50% risk of being carriers if the mother is healthy. Sex-linked recessive disorders include hemophilia, fragile X syndrome, most neuromuscular dystrophies (currently, >900 neuromuscular dystrophies are known), and hundreds of other diseases. Sex-linked dominant disorders include Rett syndrome, incontinentia pigmenti, pseudohyperparathyroidism, and vitamin D–resistant rickets.
Single gene defects
PGD is used to identify single gene defects such as cystic fibrosis, Tay-Sachs disease, sickle cell anemia, β-thalasshemia and Huntington disease.
The last group includes chromosomal disorders in which a variety of chromosomal rearrangements, including translocations, inversions, and deletions. Some parents may have never achieved a viable pregnancy without using PGD because previous conceptions resulted in chromosomally unbalanced embryos and were spontaneously miscarried.
Preimplantation genetic screening (PGS)
This term applies when the genetic parents are known or presumed to be chromosomally normal and their embryos are screened for aneuploidy ( having too many or too few chromosomes).
Primary candidates for PGS can include the following:
• Women of advanced maternal age
• Couples with history of recurrent pregnancy loss
• Couples with repeated IVF failure
• Male partner with severe male factor infertility
These patient populations are at risk of failure with IVF because of a high proportion of aneuploid embryos.