Comprehensive Chromosomal Screening (CCS) of embryos can be performed using biopsy techniques that evaluate all of the 23 pairs of human chromosomes contained within each embryonic cell. Using this advanced Pre-implantation Genetic Diagnosis (PGD) technology, chromosomally normal embryos can be distinguished from nonviable and diseased ones and then prioritized for transfer.
CCS and other PGD-based screenings are most frequently used in the following situations:
- When the woman providing the egg is “older” and therefore at relatively high risk of treatment failure or pregnancy associated with miscarriage or birth defect. Such women have often already suffered previous miscarriages or numerous failed fertility treatments for reasons that may not yet be clear.
- When a couple is at risk for having a baby with a specific genetic abnormality due to genes that either one (or both) of the genetic parents carry.
- When the parent(s) want to know the gender of each healthy embryo prior to transfer in order to pre-determine the sex of their child(ren).
- When single embryo transfer is planned to minimize the risk of multiple pregnancy.
Embryos conceived by in vitro fertilization (IVF) in the past were only assessed microscopically and the “best looking” ones were chosen for transfer. However the correlation between how an embryo looks and whether it is chromosomally normal is not always perfect. CCS solves this dilemma by actually screening the genetic information contained within the embryo prior to deciding which embryos to transfer. If an embryo has an extra or missing chromosome, and is transferred it will usually not implant, but if it does, either a miscarriage or impaired child can result. Therefore identifying these embryos in advance can be extremely helpful.
Embryos can be biopsied either at the 8-cell stage (day 3) or at the blastocyst stage (day 5 or even day 6) and then either be transferred fresh, or frozen for later use. Due to the time required to complete the DNA testing (36-48 hours in most instances) biopsies have traditionally been done at the 8-cell stage (day 3) to allow for the DNA results to come back and still permit a fresh transfer on day 5. However one potential drawback of a day 3 biopsy is that genetic abnormalities sometimes self-correct between day 3 and 5, so that false-positive results can lead to “usable” embryos being deemed ineligible for transfer. A newer option advocates testing the embryos at the blastocyst stage and then freezing them until the DNA test results are back. These biopsies are considered less invasive because only the most outer portion of the embryo (which by now has 100-150 cells) is actually biopsied. They also run a smaller risk of false-positive findings and because more than one cell from each embryo is tested, accuracy is increased. However with blastocyst biopsy a fresh embryo transfer is not always possible (since the test results may not be available in time) and instead frozen embryo transfer is required at a later date.
In the past few years more and more patients at CT Fertility have elected to create their embryos and genetically test them before making any decisions regarding embryo transfer; or at least to use the CCS results to decide which embryos to store and use for subsequent sibling projects. This “Test and Hold” option involves doing a fresh day 5 embryo transfer (which will result in a pregnancy most of the time) and then biopsying and freezing all the remaining normal appearing embryos. We then wait for the DNA results and store for future use only those that prove to be genetically normal. This enables you to increase the likelihood of pregnancy (especially when transferring only one embryo) and decrease the chance of miscarriage or other pregnancy abnormalities. Because such chromosomal abnormalities are actually very common (demonstrated in over 80% of biopsied day 3 embryos from women over 40 years old, and even approximately 50% of day 3 embryos from healthy egg donors) CCS can be a powerful tool to identify those embryos that will never result in a successful pregnancy.
In summary CCS and other PGD / embryo biopsy techniques have the potential to:
· Maximize the likelihood of success in fewer attempted cycles
· Make single embryo transfer more productive
· Decrease the chances of having to secure a new donor or carrier, and
· Permit gender selection for those families looking to determine whether their next child will be a boy or a girl.
CT Fertility offers CCS and other PGD-based options and will certainly work closely with you to decide with you which approach best suits your needs and maximizes your chances of achieving your goals – both immediate and long-term.
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