Joaquín Rueda, MD, PhD
Director of the Genetics Unit at UR Vistahermosa, UR Vistahermosa
Donor Eggs, Embryo Implantation, Failed IVF Cycles, Genetics PGS / PGT-A, Miscarriages and RPL
We’ve all probably heard about DNA (Deoxyribonucleic acid) but might wonder how it relates to embryonic health and implantation chances. DNA is the hereditary material in humans, it’s very simplistically, our genetic makeup and is found within the nucleus of each cell, where the molecule is packaged, into thread-like structures, which are called chromosomes.The majority of our cells will each have two sets of chromosomes, creating a total of 46, and these are called diploid cells. Gametes (eggs and sperm), on the other hand, only have one set of chromosomes, so just 23 in total, and are haploid cells. In most instances, when these haploid cells (the oocytes and sperm) meet, fertilisation will take place and a new cell, of 46 chromosomes, is created; a zygote (the cells created from the initial union of the egg and sperm). These cells then divide, repeatedly, and go on to become an embryo.
Almost all of us will carry normal chromosomes, however, sometimes there are issues. Gametes can be abnormal and contain an unusual number of cells; this can be seen in both male and female factor infertility.If abnormal gametes are used, in conception, the zygote is at risk of developing an irregular number of chromosomes, which then has the potential to cause embryonic developmental abnormalities, this is medically referred to as aneuploidy. Dr Rueda advises that these aneuploid embryos are thought to be the main reason for implantation failure and early pregnancy loss. He goes on to explain how the risk of aneuploidy increases with a woman’s age, and how the issue is likely to lie within the oocytes themselves, rather than actually with the woman’s ability to carry a child, which is why donor eggs are a viable option for some. However, if this is indeed the case, why then would PGS be required when using donated oocytes from younger, healthy and fertile individuals? Dr. Rueda goes on to explain how even eggs collected from donors may have abnormalities, as aneuploid embryos are potentially more common in IVF. This is due, in part, to the collection of eggs following ovarian stimulation, where the high doses of gonadotrophins (ovarian stimulation hormones) used during the stimming process increases the risk as does the medical cultivation of the pregnancy and manipulation of the gametes such as changes to temperature and oxygen in the laboratory environment. Donor eggs, therefore, can still be affected. The objective of any cycle of IVF is to transfer a healthy embryo for implantation and the use of PGS allows embryologists to be able to select the most viable. Screening helps to detect any embryos which are shown to carry genetic anomalies, enabling the transfer, or freezing, of only those embryos which have high reproductive potential. PGS is, put simply, a biopsy and genetic analysis of the embryo, after fertilisation has taken place, but before the transfer is carried out. For patients with an advanced maternal age, a history of recurrent pregnancy loss, repeated implantation failure and where severe male factor is known, Dr Rueda’s advice would be that PGS could make a difference and be used to assist in the creation of healthy offspring for couples who carry genetic diseases. He also advises that even with patients who have a good prognosis, or those who are using donor eggs genetic analysis of embryos can be helpful as there may still be the possibility of adverse pregnancy outcomes. Preimplantation genetic screening should, hopefully pave the way for more single embryo transfers to take place whilst also helping to lower the timescale it might take for couples to transfer an embryo which has higher scientific odds of implanting and progressing to become a full-term pregnancy resulting in a live birth. Dr Rueda advises that more research into preimplantation genetic screening and diagnosis in donor gametes is currently taking place. Check the clinic profile of PreGen which is Genetics Unit at the above mentioned Hospital UR Vistahermosa and employs highly specialized team of fertility doctors and embryologists. PreGen clinic has been in operation for over 30 years ago and possesses broad experience in the field of ART.