What is Preimplantation Genetic Diagnosis?
Preimplantation Genetic Diagnosis (PGD) is a prenatal method used for genetic diagnosis in early embryos before they are transferred to the woman’s uterus. It consists of performing a genetic exam on biopsied embryo cells in order to determine possible chromosomal or gene alterations. Following the exam, only the healthy embryos are chosen for transfer to the woman’s uterus.
With women or couples who are at risk of passing down chromosomal or genetic alterations to their offspring, a PGD can alert us about the state of each single embryo, thus allowing us to choose only those embryos deemed healthy enough for uterine transfer.
PGD is a complex process and cannot be performed for every single disease. The law regulates which diseases and conditions are allowed to be screened for, as well as which ones are not. Some conditions and diseases require us to request special permission from the National Commission of Assisted Human Reproduction (Spain) which will then determine whether or not the PGD treatment can be performed.
When is Preimplantation Genetic Diagnosis Recommended?
PGD is recommended when the embryos are at risk for serious alterations in the number or structure of chromosomes. It is also recommended when there is a chance of a gene alteration which could be the cause of an inherited genetic disease. At the same time, PGD is recommended when there are serious doubts about the viability of the embryos based on family history.
The Preimplantation Genetic Diagnosis Procedure
PHASE I: PRELIMINARY INFORMATION
In this phase we will perform a genetic screening in order to screen for the diseases that could be passed down from the parents. Here we strive to gather as much information as possible prior to starting the PGD.
PHASE II: OBTAINING THE EMBRYOS
Although the parents may not have any kind of problems conceiving, the embryos will be obtained by means of an In Vitro Fertilization treatment.
PHASE III: EMBRYO BIOPSY AND GENETIC ANALYSIS
The biopsy can be performed at one of two possible stages of embryo development:
- On the third day following fertilisation, when the pre-embryo is in the 6-8 cell stage. This involves removing one or two cells from the pre-embryo without compromising its normal development.
- On the fifth or sixth day following fertilisation, once the pre-embryo has reached blastocyst stage. This involves removing several cells from the trophectoderm of the pre-embryo. In this case, the pre-embryos must be frozen once they have been biopsied due to the amount of time required to obtain the genetic-chromosome results.
The biopsy that has been obtained will be sent to a Genetics Reference Laboratory where it will be processed using Array CGH, PCR, Next-Generation Sequencing (NGS) techniques. We will receive an analysis and diagnosis of each embryo that was biopsied.
PHASE IV: EMBRYO TRANSFER
Once we have received the results of the genetic analysis we will decide which embryos are going to be transferred based on their genetic-chromosomal make-up and characteristics of embryo viability.