The Benefits and Pitfalls of PGS
Posted on October 1, 2004 by Inception Fertility
 You may have heard about Preimplantation Genetic Screening as a technique provided in the IVF laboratory, and may have wondered if this technology is one you should consider incorporating in your IVF cycle. When considering various technologies in your IVF cycle, it is always important to clearly define what information you wish to gather with this technology, and also understand the pitfalls of the technology. We have two methods of screening embryos. The first is called Preimplantation Genetic Diagnosis, for couples that have a known and defined genetic disease (e.g. Cystic fibrosis, Huntington's disease, thalassemia), or are carriers of a single chromosome abnormality (chromosomal translocation). In this case we screen the embryo(s) for that particular genetic disorder, and transfer appropriate embryos. For this type of genetic screening, the aim is to conceive a healthy, unaffected child. The other type of genetic screening is called Preimplantation Genetic Screening, where we screen the embryos for abnormalities in chromosome number. We all have 23 pairs of chromosomes. Embryos that have extra or missing chromosome(s) (aneuploid embryos) are much more likely to not implant, or to produce a miscarriage. The incidence of implantation failure, or of miscarriage, depends on which chromosome(s) are missing or duplicated. We therefore can screen an embryo with a "five or nine chromosome panel." At PFC we utilize the nine chromosome panel. We look at the nine chromosomes that have been identified as most commonly being associated with implantation failures or miscarriages to see if that particular embryo has the correct number of those nine chromosomes. If so, this embryo is deemed "normal", and can be transferred back to the uterus. So who might consider PGS? Patients who have had a number of failed IVF cycles (documented failed implantations), those with a poor response to ovarian stimulation or those with poor embryo development (poor responders), those with recurrent miscarriages (>2 first-trimester miscarriages), those with a prior aneuploid pregnancy, those who are at least 35 years old are all candidates for PGS. The chances of improved pregnancy rates with PGS are dependent on the indication for PGS. When we started doing PGS for various indications, we expected a dramatic improvement in implantation rates, and therefore pregnancy rates, as we were transferring pre-selected embryos. As it turns out, we have not necessarily seen those expected improvements in all patient groups. Patients who are younger than 35 yeas of age have a better chance at improved implantation and pregnancy rates using PGS. Improvements can still be obtained for older patients, if the 9 chromosome probe set is used (some centers use a 5 chromosome panel). Studies now indicate that patients who have at least 6 fertilized eggs to screen will also have a better prognosis than those with 5 or fewer. For those patients who have five or fewer fertilized eggs in their IVF cycle, we may actually recommend not proceeding with the PGS. In this case less manipulation of embryos may provide the patient with the best overall chance at pregnancy. Patients who have had less than 3 failed IVF cycles may have greater benefit from PGS than those with > 3 failed cycles. Patients with a prior aneuploid pregnancy or with recurrent pregnancy losses can also expect an improvement. For patients who have had repetitive IVF cycle failures, or repetitive pregnancy losses, a PGS cycle may be diagnostic (explain if those failures/losses are from a high number of abnormal embryos), and in that sense may provide important information that explains those fertility failures. With those answers, the patient can then decide about pursuing similar treatment cycles, or choosing other options (using a donor egg, pursuing adoption, or choosing to live child-free). Studies indicate that results from one PGS cycle are indeed predictive of probable results in subsequent PGS cycles. In other words, if we have a cycle with a higher than expected percentage of abnormal embryos, we have to anticipate that we will probably have a similar result in subsequent PGS cycles. There are many proposed reasons to explain why we are not achieving a higher implantation/ pregnancy rate in PGS cycles. There clearly is added stress placed on the embryo(s) when one cell is biopsied out, and when the embryo is kept in culture for an extra day or two while waiting for the results of the genetic testing. We currently can only test for 9 chromosomes, and it is possible that there may be undiagnosed abnormalities on one of the untested chromosome pairs. There is also a small possibility that an embryo we deem "normal" may actually not be normal (false negative result). It also may be that simply looking at chromosomes is not the final answer. Most likely the integrity and health of the cytoplasmic structures, and other important structures of the egg are also critical in the ability of the embryo to develop into a viable and healthy pregnancy. **Who Benefits Most?**- Patients with < 3 failed cycles, and > 5 fertilized eggs
- Patients 35 year and older (if using a 9 chromosome panel)
- Patients with a history of recurrent pregnancy losses
- Patients with a previous aneuploid pregnancy
- Patients using PGS as a Diagnostic Tool for: - Repeated IVF failure - Non-obstructive Azospermia
So, while PGS is a wonderful tool that can be incorporated into the various techniques of your IVF cycle, you need to be aware of the strengths and limitations of PGS testing. Your physician can help guide you in terms of the appropriate use of PGS and whether you may benefit from incorporating PGS in your IVF cycle. Categories
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