Cytoplasmic halo characteristics during fertilization and their implications for human preimplantation embryo development and pregnancy outcome

•Halo presence correlated with preimplantation development.•Halo morphokinetics before 1st cell division positively correlated with blastocyst formation.•Halo morphokinetics could be predictive of embryo competence for transfer success. Is the spatiotemporal phenomenology of the cytoplasmic halo during fertilization related to embryonic competence? Time-lapse images from 1009 zygotes were retrospectively analysed from 560 patients who underwent IVF with minimal stimulation and single vitrified–warmed blastocyst transfer between April 2017 and March 2018. Halo presence and morphokinetics were monitored and compared relative to embryo quality, blastocyst expansion and ongoing pregnancy. Halo was observed in 88% of fertilized oocytes. Embryos derived from zygotes without halo had significantly higher rates of rapid cleavage (P = 0.0004), cell fusion (P = 0.0028) and asymmetrical division (P = 0.0002) compared with those derived from zygotes with halo. Multivariate logistic regression analysis had significantly higher developmental rates compared with the expanded blastocyst stage in embryos displaying a halo, regardless of its distribution (adjusted odds ratio 0.435; P = 0.0004). Prolonged halo time intervals were significantly correlated with increased asymmetrical division at first cell division (P = 0.0412, P = 0.0088, respectively) and decreased developmental rates to expanded blastocyst stage (P = 0.0062, P = 0.0020, respectively). Additionally, prolonged presence of the cytoplasmic halo was associated with a decreased ongoing pregnancy rate (adjusted odds ratio 0.871; P = 0.006). Poor sperm quality and decreased oocyte diameter were correlated with absence of the cytoplasmic halo (P = 0.0477, P < 0.0001, respectively) or prolonged halo presence (P = 0.0139, P = 0.0002, respectively). Halo presence and morphokinetics are associated with cleavage patterns, development to blastocyst stage and ongoing pregnancy rate after single blastocyst transfer. Halo morphokinetics seems to reflect sperm and oocyte quality. Cytoplasmic halo might be valuable predictor for refining selection of more developmentally competent blastocysts.