Find out more about blastocyst culture and the difference between day 3 and day 5 embryo transfer from the educational webinar organized with IVF Zentren Prof. Zech – Pilsen. Our presenter and expert was Dr. Renáta Krmíčková, Ph.D., a specialist in gynaecology and obstetrics with particular reference to reproductive medicine.
Day 3 or day 5 (blastocyst culture) embryo transfer?
It’s a common opinion, amongst infertility patients, that “blast is best” when it comes to transferring an embryo, but is that really the case and, if so, how much difference can a blastocyst transfer actually make to a positive outcome? In this webinar, Dr Renata Krmickova, specialist in gynaecology and reproductive medicine at IVF Zentren Prof Zech, in the Czech Republic, outlines the scientific reasons why five-day-transfers are more likely to result in a pregnancy when compared to embryos transferred on day three.
IVF is used to create a viable embryo, for transfer, under laboratory conditions. Once an egg has undergone fertilisation, either by IVF or ICSI, it is then cultivated, in an incubator, and monitored by the clinic’s embryology team. For an embryo to develop, the sperm needs to fertilise the egg, and the cells created by this process need to split and multiply.
In natural conception, a fertilised egg would usually arrive at the uterine cavity at blastocyst stage, when a woman’s endometrium is at its most receptive to receiving an embryo. Upon reaching the uterus, embryos begin the hatching process and implantation happens around days six and seven.
Choosing an embryo with the highest implantation potential
Dr Krmickova advises that one of the biggest challenges, in the IVF process, is choosing which embryo has the highest implantation potential. Across the fertility field, various selection criteria and morphological parameters have been devised which help assess the quality of each embryo, these include; Pronuclear (PN) scoring, fragmentation, and how the cells are dividing. Embryologists are then able to use this knowledge to predict which embryo is most likely to have the best developmental potential.
Day 3 embryo transfers
Day 3 embryo transfers also increase the risk of selecting incompetent embryos too; what is indicated after just 72 hours of development doesn’t always provide embryologists with the correct information. Embryonic quality, at cleaver (up to day-three) stage, is not a clear predictor for pregnancy success, this is due to the fact that, on average, around 50% of all cleaver stage embryos will never even reach the blastocyst stage. Five-day transfers truly represent the strictest form of embryo selection, with the most certain predictions.
When looking at embryonic health, the parameters which suggest a poorer quality include; more than 20% of fragmentation, embryos which are either too slow or too quick in cleaving and those with uneven blastomeres (a type of cell produced by cleavage division). Embryologists use these as a guide to help with the selection process. However, Dr Krmickova explains that even embryos displaying poor quality traits, at day three, may have the potential to continue cultivating well, going on to become blastocysts that are suitable for either transfer or vitrification.
Transferring at the blastocyst stage also allows embryologists to take the paternal factor into account. Early stage embryos are impacted by maternal influences and it is only as they progress that any possible paternal impairments can be discovered; in general, poor spermatozoa lead to poor blastocyst formation.
Dr Krmickova explains that couples who require PGS (pre-implantation genetic screening) on their embryos, should have them cultivated to blastocyst stage, stating that, a blastocyst biopsy (such as PGS) is the only reliable method to check for the possibility of aneuploidies (the presence of an abnormal number of chromosomes in a cell) or gene mutations. Aneuploid embryos are thought to be one of the main reasons for implantation failure and early pregnancy loss, the use of PGS helps embryologists select only chromosomally normal, euploid embryos for transfer, thereby increasing the odds of a successful pregnancy outcome for the patient.
Blastocyst transfers are also associated with lower ectopic pregnancy rates too, when compared to embryos transferred on day three. This figure decreases again when looking at frozen blastocyst transfer data where, on average, only 1.1% of cycles will result in an ectopic pregnancy; a figure which is similar to that found in natural conceptions.
When it comes to an embryo transfer, Dr Krmickova advises that the goal, for any medical team, should be to transfer one, high quality blastocyst. Over the last decade, research has shown that an increase in single blastocyst transfers has, in fact, led to an improved IVF outcome.
Finally, as blastocyst culture is a clinical procedure, performed in a laboratory, it is imperative that patients engage an experienced team for their IVF procedures. Whilst evidence supports the understanding that day-five transfers do lead to an increased chance of pregnancy, the skills of the embryologist are key, as are the laboratory equipment and conditions.