How does preimplantation genetic testing work?
Preimplantation genetic testing (PGT) is available for couples undergoing IVF. The steps of the IVF process include:
- Medications are used to suppress a woman's natural menstrual cycle.
- Her ovaries are then stimulated with medications to produce multiple follicles, each of which may contain an oocyte (egg).
- The eggs are retrieved from the woman's ovary by a needle placed in the vagina.
- In the lab, the eggs are combined with the male partner's sperm in a special culture medium that allows fertilization and the growth of high–quality embryos.
Embryo biopsy can be performed after three days of culture in the laboratory. The embryos are typically eight-cell embryos on Day 3, and the process involves the removal of one to two cells.
After the biopsy and following receipt of the results from the genetic/chromosomal testing, embryo(s) of the best quality that are not affected by the genetic disorder or chromosomal abnormality) are selected for transfer to the uterus. For Day 3 embryo biopsies, the embryo is usually transferred "fresh" following two additional days of culture in the laboratory (Day 5 embryo transfer).
In some cases, the biopsy will be done on either Day 5 or 6 (trophectoderm biopsy). At this stage, the embryo consists of many cells and is called a blastocyst. Cells are removed from the outer layer of cells called the trophectoderm.
Following the biopsy of a good quality blastocyst, the blastocyst is frozen. When the patient receives the results from the genetic testing, the non-affected or chromosomally normal blastocysts are thawed and transferred in a subsequent frozen embryo cycle.
Embryos are analyzed by one of the following techniques:
- Testing for heritable genetic disorders: We conduct this type of testing when a couple has a significantly increased risk of conceiving a child with a severe genetic disorder. The test involves performing a polymerase chain reaction (PCR) on the biopsied cell(s) to determine the presence of a single gene. The cells obtained at biopsy are loaded into a tiny tube of medium and sent to for analysis. We amplify the specific area of DNA of interest by making thousands of copies of the DNA through repeated cycles of DNA strand separation and replication. The sample can be analyzed for the presence of a specific sequence of DNA or gene and also for linkage markers near the gene. The biopsied cells are destroyed during this process, so they can't be used for another purpose or returned to the embryo.
- Testing for chromosomal abnormalities: During this test, we amplify the genetic material (DNA) within the biopsied cell(s) using a technique called the polymerase chain reaction (PCR). This amplification produces enough DNA to use a second technique, known as array comparative genomic hybridization (aCGH). Array CGH assesses the amount of DNA derived from each chromosome, revealing whether or not there are both a normal amount and correct number of chromosomes. Just as in the testing for heritable genetic disorders, the biopsied cells are destroyed during this process and can't be used for another purpose or returned to the embryo. aCGH can be used to screen for numeric abnormalities in all chromosomes and/or known rearrangements of chromosomes (translocations). Array CGH doesn't detect all types of chromosome aberrations or genetic mutations and cannot distinguish between no translocation present and balanced translocation present.
The results of preimplantation genetic diagnosis will be reported to you no later than the morning of your scheduled day for embryo transfer. Typically, this is five days after oocyte retrieval and in vitro fertilization are performed. Of the embryos that are not affected by the genetic disorder or chromosomal abnormality, the best quality embryos are selected for transfer to the uterus. If additional unaffected and good-quality embryos are available, they may be cryopreserved for a future embryo transfer.