Preimplantation Genetic Diagnosis (PGD) or PGS (Screening) refers to testing that is performed on embryos prior to transfer to the uterus or on oocytes prior to fertilization.

Some of the common reasons for PGD are specific single gene conditions. PGD is available for almost any inherited condition for which the exact genetic mutation is known. 

PGS is used in the field of assisted reproduction for aneuploidy screening and diagnosis of unbalanced inheritance of chromosome abnormalities (translocations).
 

Indications

Who should consider having PGD/PGS?

• Women of advanced maternal age (over 35)
• Couples with recurrent miscarriages that are otherwise  unexplained
• Couples who have experienced several failed IVF cycles
• Couples who have had a prior pregnancy with a chromosome abnormality
• Men with infertility requiring intracytoplasmic sperm injection (ICSI)
• Men with positive aneuploidy sperm screening
• Couples where at least one partner has aneuploidy mosaics
• Couples where at least one partner is a carrier of an X-linked disease but the exact genetic mutation is not known
• Couples where at least one partner is a carrier of a structural chromosomal rearrangement
• Couples who desire to have a child that would provide a bone marrow match for sibling
   

These indications can be categorized differently as:

• PGD for single gene mutations
• PGD for determination of chromosome number (aneuploidy)
• PGD for structural chromosome abnormalities (translocations)
• PGD for Tissue Typing

Technical aspects of PGD

PGD utilizes IVF often with ICSI, where multiple eggs are matured and retrieved; the oocytes are fertilized and the resulting embryos are grown in culture until the 6-8 cell stage (day 3 of embryo development). At this point, the embryo is biopsied with the removal of 1-2 cells.

The isolated cells are evaluated for the specific genetic condition. 

In order to obtain results of the biopsy, the cells removed must contain a nucleus, as the nucleus contains the genetic information necessary for testing. Embryos that are determined to be unaffected by the genetic mutation (PGD) or are chromosomally balanced (PGS) are transferred back to the woman's uterus on day 5 of embryo development.
 

How are embryos chosen for transfer?

Embryos that have both a normal test result and have continued on in development may be transferred.  Sometimes embryos that have normal genetic tests will have a physical problem that prevents them from growing normally. Sometimes embryos that have abnormal genetic tests will appear to be physically normal. The combination of normal genetic testing with normal physical appearance indicates the highest chance of becoming a healthy pregnancy.