What Is PGD or PGS?

If you are just beginning to try to conceive using assisted reproductive technology you may be confused by the process of preimplantation genetic screening (PGS) or preimplantation genetic diagnosis (PGD).  This confusion wouldn’t be uncommon as the process involves a series of very complex biological steps. 

In vitro fertilization is the first step involved, as the cells can only be retrieved from the embryo while it remains growing in the laboratory.  A biopsy is taken from each embryo at either day 3 or day 5 of growth.  Day 3 embryos consist of 6-10 cells, in comparison to day 5 embryos (blastocysts) which consist of approximately 200 cells.  When an embryo is biopsied on day 3 only one or two cells are removed, while a day 5 embryo may have several cells removed from it increasing the amount of DNA available for analysis. 

A study done by the Reproductive Medicine Associates of New Jersey was published in October 2011 comparing the safety and efficacy of embryos biopsied for genetic screening at day 3 and day 5.  Day 3 embryos had a significantly reduced chance of developing into a healthy pregnancy.  Day 5 embryos biopsied the trophectoderm which is the layer of the embryo responsible for turning into the support network called the placenta.  This allowed the cells responsible for development of the baby to remain untouched. 

Until recently, PGS/PGD was typically performed on day 3 embryos through fluorescent in situ hybridization (FISH).  FISH tests by placing small pieces of DNA matching the chromosomes being tested.  With FISH only 10-12 of the 46 chromosomes may be analyzed, including X, Y, 1, 13, 16, 18 and 21. 

However, fertility doctors have moved toward biopsy of day 5 embryos with the use of comparative genomic hybridization (CGH) or single nucleotide polymorphism (SNP).  Both CGH and SNP allow for all 23 chromosomal pairs to be tested providing a detailed picture of the chromosome.  This may detect an imbalance of chromosomes called a translocation. 

FISH, CGH or SNP all allow for the gender of the embryo to be known prior to returning it to the uterus. 

Single gene disorders may be analyzed through polymerase chain reaction (PCR).  This test targets specific genes generally known to be abnormal in one or both of the genetic parents.  It cannot be used for gender selection.  (Gender selection allows for parents to select the desired sex embryo for family balancing or to avoid a sex linked trait).