UNASSIGNED : The presence of metaphase II (MII) spindle together with the polar body (PB) indicates completion of oocyte maturation. This study was designed to explore if spindle imaging can be used to optimize timing of intracytoplasmic sperm injection (ICSI). METHODS : The study involved 916 oocytes from 234 conventionally stimulated ICSI cycles with an unexpectedly poor ovarian response. All PB-displaying oocytes were subjected to polarized light microscopy (PLM) prior to ICSI. When MII spindle was absent in the majority of oocytes, ICSI was postponed and performed after additional spindle imaging. Fertilization, embryo development, and clinical outcome were evaluated with respect to the observed spindle pattern. RESULTS : The visible spindle was absent in 32.64% of PB-displaying oocytes. The late-maturing oocytes extruding PB in vitro were less likely to exhibit a spindle signal than in vivo matured MII oocytes (38.86% vs. 89.84%). When fertilization was postponed, 59.39% of initially spindle-negative oocytes developed detectable MII spindle. Spindled eggs had significantly higher developmental potential, and the presence of the spindle has been identified as an independent measure for predicting the formation of the blastocyst. Embryos derived from spindle-positive oocytes also showed a higher chance to implant and develop to term. Notably, 11 children were conceived by finely timed fertilization of late-maturing oocytes which are normally discarded. CONCLUSIONS : The study confirms the prognostic value of spindle imaging and demonstrates that immature oocytes can be clinically utilized and give rise to live births when the timing of ICSI is adjusted to their developmental stage.