When undergoing in-vitro fertilization, it is possible to biopsy the developing embryo and obtain obtain one or more cells for genetic analysis prior to transferring the embryo into the uterus. This technique, when performed to determine if an embryo has a specific genetic disorder, is called preimplantation genetic diagnosis (PGD).
This technique enables couples at risk for specific genetic diseases to determine which embryos may carry the disease and which do not. In this way, couples are able to transfer only non-diseased embryos and insure that the resulting child will not have the disease in question.
Couples are determined to be at risk for a specific genetic disease by having a previous child with the disease or having a genetic screening test on both members of the couple that shows their child to be at risk. Examples of common genetic diseases that can be avoided via PGD include Huntington’s Disease, spinal muscular atrophy, cystic fibrosis, Tay Sachs Disease, Sickle Cell anemia, and Fragile X syndrome. Additionally, some diseases are known to mainly occur in one sex, and the use of PGD for selection of embryos of a certain sex can help decrease the chances of a diseased child.
The most common cause for miscarriage is an abnormal number of chromosomes in the fetus. By analyzing the number of chromosomes present in one or more cells obtained from each embryo, selection of only those embryos with the correct number of chromosomes can be accomplished. This technique, called preimplantation genetic screening (PGS) can result in a decreased chance of miscarriage for a woman undergoing embryo transfer.
Both PGD and PGS can be performed on biopsies from day 3 embryos (blastomere biopsy) or day 5/6 embryos (trophectoderm biopsy). The standard for many years was to biopsy the embryo on day 3 and extract a single cell, which was then sent to the genetics laboratory for analysis. The information could be obtained in 48 hours, and embryos could be chosen for transfer at that time (a fresh, day 5 or 6 transfer).
Recently, however, biopsy techniques have improved sufficiently to make day 5/6 biopsies safer and more accurate. Combined with improved techniques to freeze embryos (vitrification), this latter biopsy method has now become the primary technique used. Today, for nearly all PGD and PGS, embryos are grown to the day 5/6 (or blastocyst) stage and the outer layer of cells, destined to become placenta, are biopsied. The embryos are then frozen and genetic results obtained in a less frantic manner. Once it is known which embryos are unaffected or chromosomally normal, plans can be made to replace them in subsequent “frozen-thawed” embryo transfer.
The Wisconsin Fertility Institute was the first center in Wisconsin to primarily use this new paradigm in preimplantation genetics. It is our belief that in the next few years preimplantation genetic screening and exclusion of abnormal embryos will become a standard aspect of in-vitro fertilization treatment, helping to improve pregnancy rates while simultaneously decreasing the rate of miscarriage.