Preimplantation Genetic Diagnosis (PGD) is a sophisticated genetic testing procedure performed on embryos created through In Vitro Fertilization (IVF) before they are transferred to the uterus. This advanced technique is primarily utilized by couples who are at risk of passing on inherited genetic conditions to their offspring. While often associated with couples who have a known family history of a specific condition, PGD can also be relevant for couples undergoing fertility treatments who are identified through routine genetic screening as carriers of genetic disorders.
PGD offers a proactive approach to family planning for individuals concerned about hereditary diseases. It is applicable to virtually any inherited condition where the precise genetic mutation is known. It’s important to note that the process requires the development of a customized genetic test tailored to the specific needs of each couple. This test development phase can take several months and must be completed prior to the commencement of an IVF cycle.
The PGD process is intricately linked with IVF. It begins with ovarian stimulation to mature multiple eggs, followed by egg retrieval. These eggs, technically termed oocytes or primitive egg cells, are then fertilized by injecting a single sperm directly into each egg using a technique called intracytoplasmic sperm injection (ICSI).
The resulting embryos are carefully cultured in the laboratory until they reach the six-to-eight-cell stage, typically around day three of development. At this stage, a crucial step known as embryo biopsy is performed. This involves the removal of one or two cells from each embryo. Remarkably, this delicate procedure does not harm the remaining cells within the embryo, ensuring its continued viability.
These biopsied cells are then meticulously analyzed to detect the presence of specific genetic conditions. Embryos identified as unaffected by the condition under scrutiny are selected for transfer back to the woman’s uterus, usually on day five of embryo development.
Two primary molecular techniques are employed for genetic assessment in PGD:
- Polymerase Chain Reaction (PCR): PCR is a powerful technique that amplifies specific DNA sequences. It works by creating numerous copies of a particular gene of interest. This amplification process is essential as it allows for the detection of even minute amounts of DNA, enabling accurate diagnosis of genetic conditions.
- Fluorescent In Situ Hybridization (FISH): FISH is a cytogenetic technique used to count the number of chromosomes within a cell. It is particularly valuable for identifying abnormalities in chromosome number, such as in cases of Down syndrome, or structural chromosomal defects like translocations.
For couples considering PGD, it is crucial to seek out centers with proven expertise. At institutions like UCSF, embryology laboratories are staffed by professionals with extensive experience in embryo micromanipulation and biopsy. Furthermore, genetic counselors play a vital role in coordinating the PGD cycle, working closely with both the IVF team and the PGD laboratory to ensure a streamlined and supportive experience for patients navigating this advanced reproductive option.
