Overview of Zygote transfer and cleavage stage embryo transfer

In the IVF process, the uterine transfer of embryo is usually taking part after the created embryo in the laboratory set-up reached the blastocyst stage. There are three distinct phases appear during the embryo creation processes in the laboratory set-up after the egg retrieval process, which are pronuclear stage occurs at the day one, then the cleavage stage started at the day second to the third day and subsequently blastocyst stage appears at the fifth to the sixth day. This complex process is monitored through microscopic observations for checking the quality of the embryos during the laboratory development of the embryo.

Ongoing debates related to the best time of embryo transfer is continuing from more than two decades. Transferring embryo at the cleavage stage on the second or third day is a general practice, but recently blastocyst stage has gained popularity. However, some medical researchers challenge that the selection and implantation of the high-quality embryo can potentially achieve on the third day. But in a practical scenario, medical professionals considering the challenges lies in the viable embryo selection, not the day of embryo transfer. Recently,  pronuclear embryo transfer at the day one is also conducted on the day one by considering the pronuclear and nucleolar morphology of the created embryo. This process minimizes the cost of the overall process, due to the simplification of the laboratory process.

A previous randomized study result supported that implantation of the embryo at day one and day second or third give similar pregnancy outcome. But clinicians considered that dynamic changes in nuclear structure take some time and during day three to five morphological correction can only be possible. Therefore, early implantation at the pronuclear stage may not provide enough prediction of pre-implantation abnormality development.

Practically, there is no agreement present for optimal embryo transferring process. But continues analytical research reports do not prefer early stage zygote transfer, as they have found a vast difference between pregnancy rate in comparison to the third day of cleavage stage embryo transfer.

One of the primary cause of unproductive result of zygote transfer is no specific selection criteria or assessment methods have described for the zygote to be transferred.  In most of the cases, a number of zygotes are transferred, as some of the zygotes do not reach to final stage due to arresting of the subsequent stage. Therefore, maximum zygotes implantation gives a positive outcome, but this may increase the scope of multiple births and cause pregnancy risk. However, there is a possibility to less number of embryo transfer in the cleavage stage and possibly reduce the risk of multiple births and increase the success rate of pregnancy. The aim of embryo cryopreservation is to preserve good quality zygote. But, in the case of cryopreservation of embryo requirement for future use, the number is increased in the case of pronuclear cryopreservation, as the quality of embryo is not significant in this stage.

Therefore, the overall analysis report obtained from different medical research paper is that cleavage stage embryo transfer on day third gives a better result than zygote transfer at day one. However, the zygote scoring process is gradually developed and it has interpreted that Pattern 0 zygotes provide similar pregnancy outcome as cleavage stage embryo transfer on day third. But non‐pattern 0 morphology zygotes have not sufficient efficacy. But yet now, alone zygote scoring process is not considered as reliable prediction method to check the viability of the embryo. In this field more precise and improve zygote selection criteria need to be discovered for obtaining a successful result at day 1 transfers.

Pattern 0 zygotes had similar day 3 embryo quality and pregnancy outcome as compared to zygotes with non‐pattern 0 morphology. The present criteria used for zygote scoring alone are not reliable to predict the viability of the embryo. Further studies are required to determine whether a progressive rather than single‐point observation could improve the selection criteria for day one transfers.

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