Preimplantation genetic screening refers to the removal of one or more cells from an in vitro fertilization embryo to test for chromosomal normalcy. PGS screens the embryo for a normal chromosome number. PGS is a method of chromosome disorder screening on IVF embryos.
Preimplantation genetic diagnosis involves the following steps:
- First, one or two cells are removed from the embryo.
- The cells are then evaluated to determine if the inheritance of a problematic gene is present in the embryo.
- Once the PGD procedure has been performed and embryos free of genetic problems have been identified, the embryo will be placed back in the uterus, and implantation will be attempted.
- Any additional embryos that are free of genetic problems may be frozen for later use, while embryos with the problematic gene are destroyed.
There are two kinds of PGD / PGS biopsy:
- Trophectoderm Biopsy: Trophectoderm biopsy involves removing some cells from the trophectoderm component of an IVF blastocyst embryo. The removed cells can be tested for overall chromosome normality (PGS) , or for a specific gene defect (PGD).
- The embryo should be at the expanded blastocyst stage (or beyond) at the time of cell removal
- This stage is reached on day 5 to 6 after fertilization
- Trophectoderm cell removal is much less traumatic compared to blastomere removal
- Blastomere Biopsy: Blastomere biopsy is removal of a cell on Day 3 at the “cleavage” stage – before a blastocyst is formed. Day 3 embryo biopsy is traumatic and lowers the embryo’s potential for implantationWe generally choose to do trophectoderm biopsy.
We generally choose to do trophectoderm biopsy.
Day 5 embryo -blastocyst- just hatching
Advantages of trophectoderm biopsy and PGD on success rates
- The advantage of trophectoderm removal is that the embryo is much less traumatized by the procedure compared to blastomere removal done on day 3
- Since the embryo has many more cells (about 100) at the blastocyst stage than on day 3 (about 6-10), we can remove about 5 cells with trophectoderm biopsy with little or no impact on its potential
- Trophectoderm biopsy allows the potential to increase IVF live birth success rates by screening for chromosomally normal embryos prior to transfer back to the uterus
Disadvantages of trophectoderm biopsy
- At this point performing the genetics techniques for chromosomal or genetic analysis on the cells requires 24-48 hours
- So a day 3 biopsy can be done with the information about the chromosomes or genetics coming back in time to do transfer on day 5 with fresh embryos
- However, when the cell removal is done on a day 5 embryo the genetic or chromosomal results will not be back in time to do a fresh transfer procedure
- So we must do the trophectoderm removal, then freeze the embryos, and wait to get the genetics results back before we thaw and transfer back blastocysts to the uterus
The disadvantages of this approach
- Increased cost from adding a frozen embryo transfer cycle
- Delay of about a month before the transfer can be done
- The need for development to the blastocyst stage to do a biopsy procedure. Not all couples will have embryos develop to blastocyst stage