Whether these are embryos for freezing, or embryos for transfer, the process is the same. The embryos are assessed for their morphology and a score is assigned. The “best” embryo is then selected. There are 3 main tools used by embryologists to select the best embryo, however, only two are done on every patient. The third is a tool known as preimplantation genetic screening (PGS), which I will discuss more in depth later on.
The first tool is embryo morphology. Does the blastocyst look good? This is a very subjective tool. This is the earliest form of embryo selection in the IVF lab. It would make sense that the embryos that look the best, should result in more pregnancies. Using this logic, selecting embryos with the best morphology should lead to certain success! Unfortunately, we know all too well that isn’t always the case. Morphologically perfect embryos do not always produce positive pregnancies’, and conversely, morphologically imperfect embryos sometimes do!
The second tool is embryo development on day 5 and/or day 6. It is related to the function of the embryo. By culturing to the blastocyst stage, we can select embryos likely to be more viable based on how the embryo is functioning and developing in the lab. When embryos were routinely transferred on day 3, there was no way to know if a particular embryo had developed past that critical cellular stage and would have also stalled out in the lab, or if the embryo did have to ability to develop into a blastocyst, but did not have the capacity to implant . By culturing to the blastocyst stage, we have an idea of the functionality of the embryos. This gives our physicians far more information and knowledge about how to best treat their patient’s in order to achieve a positive outcome. Recent data (Shapiro et al.) have suggested that embryos that divide slowly and reach blastocyst stage on day 6 have a much lower pregnancy rate when transferred. However, as discussed in a previous blog, the pregnancy rate is excellent if these embryos are frozen (cryo-preserved) and transferred in a subsequent cycle.
The third and most complex tool for embryo selection is testing to see if the embryo has a normal number of chromosomes. This is science. We know that many embryos that have abnormal chromosome numbers (aneuploidy) stall in their development and do not reach the blastocyst stage. However, we know that MANY TIMES even morphologically perfect blastocysts can be chromosomally abnormal. With pre-implantation genetic screening (PGS) of embryos, we can tell if an embryo has a normal number of chromosomes. This is a test that requires the embryos to be biopsied, frozen, and the cells shipped off to a genetic lab for analysis. This is a really big deal in the reproductive world! By combining the grade and the chromosomal status of the embryo, we can transfer a SINGLE blastocyst and get very high success rates.
By combining subjectivity (the grade of the embryo) and science (the chromosomal status of the embryo) we are able to pick out the embryo most likely to implant and result in a successful pregnancy for our patient’s. We call this SMART IVF.
Liza has been an embryologist at InVia Fertility Specialists since 2002. She completed her studies, first in animal science at Southern Illinois University, followed by clinical embryology/reproductive genetics at Eastern Virginia Medical School. Her interests include reproductive genetics, fertility preservation, and mitochondrial function in the human oocyte. She loves being a part of helping others achieve and experience the joy of becoming a parent. Seeing the end result .your baby.makes all the hours in the lab worth it!