P-204 The pattern of the zygote cleavage to three cells must be considered before embryo transfer
Authors: Nemerovsky L , Ghetler Y , Wiser A , Levi M 
Published: June 2023

Abstract

Study question
What is the clinical outcome of instant irregular cleavage (IDC) from zygote to three cells?

Summary answer
IDC embryos, did not develop to blastocysts and did not acieve pregnancy; FC embryos that reached blastocyst stage exerted similar pregnancy rates as control embryos

What is known already
Evaluation of the zygote morphology is one of the earliest embryonic stages investigated. Several zygote grading systems were proposed based on pronuclei size and NPB (nucleolus precursor bodies) distribution, thus is an effective indicator of the embryo’s potential to result in a life birth. The first cleavage of a zygote is a highly coordinated event. Disruptive timing of the first cleavage, named direct cleavage (DC, less than 5 hours), is associated with formation of poor embryo quality. Early identification of pathological embryos is challenging and impacts the timing of embryo fate decision in ART.

Study design, size, duration
A total of 1978 fresh IVF cycles from single unit were analyzed and a total of 4012 embryos were studied. Only 2pn embryos, cultured exclusively in time-lapse system for 5 days were included. The morphokinetics of embryos and their outcome were recorded, from z-score analysis through day 5. We discriminated between IDC, FC and normal cleavage pattern of embryos and compared clinical outcome of various morphokinetic parameters. We also evaluated possible early predictors for IDC.

Participants/materials, setting, methods
We generated three study groups according to embryo cleavage pattern: (I) Control, normal cleavage (n = 551); (II) FC, zygote to three cells within 5 hours (n = 1587); (III) IDC, instant cleavage from zygote to three cells (n = 922). The association between z-score of 138 IDC embryos and their sibling random controls was thoroughly investigated. All time lapse annotations were performed by senior embryologists. Statistical analysis was performed by SPSS software.

Main results and the role of chance
IDC embryos were mainly arrested at day 3 and the number of usable embryos (reached blastocyst stage and were suitable for embryo transfer or cryopreservation) was negligible; 4/922 (0.4%). In comparison, the amount of usable FC embryos was 108/1587 (6.6%) and control embryos usage reached 180/551 (32.7%). While the pregnancy rate of control and FC embryos, which reached embryo transfer were similar (40.35% and 42.55%, respectively); transfer of IDC embryos did not result in pregnancy. Additionally, the timetable as measured by time of PN fading and time from fading to first cleavage, differed significantly between the three groups. Therefore, we performed a thorough analysis of zygote morphology data of IDC embryos compared to control sibling embryos in search of early possible markers for these findings. We have not detected significant differences in z-score analysis, reflected by number and size of nucleoli, as well as pronuclear symmetry.

Limitations, reasons for caution
IDC embryos number used for z-score analysis was limited since many didn’t meet the inclusion criteria (lack of overlapping of the pronuclei, presence of random of control embryo and good quality image).

Wider implications of the findings
The decision regarding the fate of IDC and FC embryos should include the pattern of first cycle cleavage. Culture IDC and FC embryos for 5 days up to the blastocyst will spare transfer of embryos that are fated to arrest even when their morphological grade on day 3 is acceptable.

Trial registration number
0043-22-MMC