High oocyte immaturity rates affect embryo morphokinetics: lessons of time-lapse imaging system

Does oocyte immaturity rate affect morphokinetic events in a time-lapse imaging (TLI) system?

Historical cohort study carried out in a private university-affiliated IVF centre. Injected oocytes (n = 3368) cultured in a TLI incubator, from intracytoplasmic sperm injection (ICSI) cycles (n = 474) carried out between March 2019 and December 2020, were analysed. The effects of immature oocyte rates (the number of germinal-vesicle and metaphase I oocytes by the number of retrieved oocytes in each cycle, on morphokinetic events) were investigated considering clustering of data using mixed models. Evaluated kinetic markers were pronuclei appearance (tPNa), timing to pronuclei fading (tPNf), timing to two (t2), three (t3), four (t4), five (t5), six (t6), seven (t7), and eight cells (t8), timing to morulae (tM) and timing to start of blastulation (tSB) and to blastulation (tB). Durations of the second (t3-t2) and third (t5-t3) cell cycles (cc2 and cc3, respectively) and timing to complete synchronous divisions s1 (t2-tPNf), s2 (t4-t3) and s3 (t8-t5) were also evaluated.

Positive relationships were observed between oocyte immaturity rates and slower tPNa, tPNf, t2, t3, t4, t5, t6, t7, t8, tSB, tB and cc3. Multinucleation at two- and four-cell stages were positively correlated with oocyte immaturity rate. The KIDScore ranking was negatively correlated with oocyte immaturity rate. No associations were found between oocyte immaturity rate and clinical outcomes.

Increasing oocyte immaturity rate correlates with delayed cell cleavage and blastulation. These findings highlight the importance of TLI for the identification and de-selection of slow-growing embryos for transfer, in cycles with high oocyte immaturity rate.