Can the pronuclei (PN) morphology and the time of PN breakdown (PNB) predict the potential of embryos to result in live birth?In comparison to embryos resulting in no live birth, PNB occurred significantly later in embryos resulting in live birth and never earlier than 20 h 45 min. None of the tested scoring systems were shown to predict the live birth outcome in a time-lapse set-up.The PN morphology is supported as a prominent embryo selection parameter in single light microscopy observations, although controversial results have been reported.This was a prospective study of 159 embryos, all of which were later transferred. The PN morphology of 46 embryos which resulted in live birth was compared with that of 113 embryos which resulted in no live birth.From 1 March 2010 to 30 August 2011, 130 couples underwent fertility treatment by ICSI. Embryo culture was performed in a time-lapse set-up from fertilization to intrauterine transfer. PN morphological assessment was performed on every embryo replaced, using six different scoring systems at different times.No embryo with PNB earlier than 20 h 45 min resulted in live birth. All six PN assessment models showed no significant distribution of scores (P = NS) between the live birth and no live birth groups at 16 h post-fertilization (PF), 18 h PF and 40 min before PNB. The outcomes of assessments changed significantly (P < 0.001) over time and the time of PNB was found to be the optimal stage to evaluate the PN morphology.The study includes only embryos reaching the 4-cell stage after ICSI, and transferred at 44 h PF.The PN morphology changes over time, indicating that the single light microscopy observation approach is deficient in comparison to time-lapse. Although the assessment of the PN morphology does not improve embryo selection, the timing of PNB should be included in embryo selection parameters.