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Genetics and Comprehensive Chromosomal Screening (CCS)
Over the past several decades, advances in reproductive technologies have provided new possibilities for couples experiencing infertility. IVF has allowed many couples to overcome infertility due to tubal disease, and ICSI has overcome most cases of male factor infertility. As women age, however, traditional IVF has not always been successful in helping physicians identify the healthiest embryo. By the time most women are in their 40’s, 90% of eggs are abnormal. Some eggs possess too many chromosomes, and others do not have enough. In any case, the abnormal embryos may either not implant, may result in miscarriages, or may even result in unhealthy babies. To prevent unhealthy embryos from being transferred and to increase the chance for individuals and couples to achieve a healthy pregnancy, we frequently biopsy embryos prior to transfer and then analyze the embryos using a form of assisted reproductive technology known as Comprehensive Chromosomal Screening (CCS).
Comprehensive Chromosomal Screening offers many benefits. It reduces the chances of miscarriage, while also decreasing the likelihood of an abnormal pregnancy. Additionally, rather than risking twins or triplets due to the transfer of multiple unscreened embryos, CCS increases the likelihood of achieving a healthy pregnancy by often allowing physicians to transfer a single screened embryo. Given the increased risks that are incurred by the mother and babies in a twin pregnancy, elective transfer of one healthy embryo is more likely to result in a positive outcome for all.
For decades, Preimplantation Genetic Diagnosis (PGD) has been used to test embryos for gene mutations. We commonly screen potential parents to determine whether they carry an increased risk of transmitting diseases such as Cystic Fibrosis and Sickle Cell, though at present, it remains technologically unfeasible to screen parents (or embryos) for the 6,000 known genetic diseases. In addition, there are other disorders not apparent in the DNA (imprinting disorders) that can also not currently be easily identified by current testing. Our team recommends that patients seek genetic counseling to best determine what they are at risk for. If a couple is at risk for transmitting a mutation to their offspring, it may be possible to test the embryo (PGD) prior to transfer or recommended to analyze the resulting pregnancy with a chorionic villus sampling (CVS) or amniocentesis.
Current CCS technology is a form of preimplantation genetic screening that specifically looks at structural issues such as chromosomes rather than genes and mutations. With a high degree of accuracy, embryos can be analyzed to determine whether they have conditions such as Down syndrome (“Trisomy” or three copies of Chromosome 21) or Turner syndrome (“45XO” or absence of one sex chromosome). An embryo that has 46 chromosomes (including two copies of chromosomes 1-22 and an XX or XY) is known as a ‘euploid’ embryo, while those that contain an abnormal number of chromosomes are known as aneuploid.
As a female’s eggs age, the risk of having aneuploidy embryos increases, and in some patients, more than half of resulting embryos may be identified as abnormal. Since over fifty percent of miscarriages are caused by having either too many or too few chromosomes, screening embryos prior to transfer significantly increases the chance of a successful implantation and a healthy ongoing pregnancy.
How Single Embryo Transfer (SET) with CCS is Accomplished
Embryo selection can be accomplished in a number of ways. Conventional methods involve microscopic evaluation, in which the embryos are given grades or scores based on morphology. New methods being evaluated include time-lapse photography to monitor embryo cell division. In addition to the morphokinetic information, Comprehensive Chromosomal Screening offers critical information regarding the chromosomal make up of embryos. Specifically, after fertilization and five days of embryo growth, the embryo reaches the blastocyst stage. At that stage, using a laser, a few cells are biopsied from each embryo and are analyzed in a specialized genetics laboratory. In some cases, the embryos are quite advanced and biopsied early, in which case results may be ready in time to transfer the fresh embryo into the uterus. Commonly, the biopsy is performed and the embryos are cryopreserved or “vitrified,” and if the embryos are found to be normal, they are thawed and transferred in the subsequent cycle. Results to date suggest that freezing and thawing the screened embryos and transferring them into a prepared uterine cavity the following month may actually improve the likelihood of a successful outcome.
The number of chromosomally normal embryos that are identified in the process varies from woman to woman, but overall, the CCS process provides information that normally results in improved ability to select healthy embryos, improved implantation rates, and reduction in miscarriage and multiple pregnancy risk.
The Benefits of Comprehensive Chromosomal Screening
Combining in vitro fertilization with Comprehensive Chromosomal Screening offers benefits to many patients. Whether a couple has unexplained infertility, recurrent miscarriages, age related reproductive changes, desires a child of a specific gender, or wants to select the single best embryo for transfer, CCS may be an appropriate choice. Technology continues to provide physicians with new tools to offer patients in efforts to improve success rates and therefore allow positive outcomes for those seeking parenthood.