During vertebrate oogenesis, the prophase of the first meiotic division is extremely long – it lasts 4-8 months in Xenopus oocytes –to allow the eggs to grow and accumulate all the components necessary for early embryonic development. On p. 1543, Karaiskou et al. report that a polo-like kinase 1 (Plx1) is the critical limiting factor whose absence prevents Xenopus oocytes re-entering meiosis until their growth is complete. Only oocytes at stage VI of growth are competent to resume meiosis in response to progesterone, the trigger for meiotic maturation at ovulation. In these cells, progesterone induces the activation of pre-M-phase promoting factor (pre-MPF), an inactive Cdc2-cyclin B2 complex. The small amount of active MPF formed then establishes an MPF auto-amplification loop. By overexpressing Plx1 in stage IV oocytes, Karaiskou and co-workers show that the absence of Plx1 in stage IV oocytes prevents both the generation of the active MPF trigger and the establishment of an MPF auto-amplification loop, thus preventing premature entry into meiosis.