There was no specific funding for the preparation of these proposed guidelines. Meetings were held opportunistically during scientific conferences and using online communication tools. H.N.C. is a scientific consultant for ESCO, supplier of Miri TL. I.E.A. is a minor shareholder in Unisense Fertilitech, supplier of the EmbryoScope. Full disclosures of all participants are presented herein. The remaining authors have no conflict of interest.

Multinucleation in two-cell-stage embryos is a frequent event, which is reversible in a high proportion of embryos, without impact on the implantation rate; and embryos with multinucleated blastomeres have a reduced outcome compared with those with binucleated blastomeres when multinucleation is present in four-cell-stage embryos.

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Abnormal luteal function is a common issue in assisted reproduction techniques associated with ovarian stimulation probably due to low levels of LH in the middle and in the late luteal phase. This defect seems to be associated with supraphysiological steroid levels at the end of follicular phase. The luteal phase insufficiency has not got a diagnostic test which has proven reliable in a clinical setting. Luteal phase after ovarian stimulation becomes shorter and insufficient, resulting in lower pregnancy rates. Luteal phase support with progesterone or hCG improves pregnancy outcomes and no differences are found among different routes of administration. However, hCG increases the risk of ovarian hyperstimulation syndrome. In relation to the length of luteal support, the day of starting it remains controversial and it does not seem necessary to continue once a pregnancy has been established. After GnRHa triggering ovulation, intensive luteal support or hCG bolus can overcome the defect in luteal phase, but more studies are needed to show the LH utility as support.