Considerable heterogeneity exists in the perifollicular vascularity and oxygen supply among follicles during ovarian stimulation, resulting in different oocyte maturation statuses in a cohort [3,14]. Oocyte maturation includes nuclear and cytoplasmic maturation [16,37]. Nuclear maturation is easily identified by the extrusion of the first polar body. However, cytoplasmic maturation of the retrieved oocyte, which is essential for embryonic development in vivo and in vitro, remains difficult to determine [12,25,32,41].

Nuclear matured (metaphase Ⅱ, MⅡ) oocytes are not always accompanied by fully matured cytoplasm but immature or over-matured cytoplasm [29,31,32]. A high proportion of nuclear immature oocytes (MI and GV oocytes) in an oocyte cohort was associated with a reduced pregnancy rate and live birth rate in ICSI cycles [28]. The immature cytoplasm of the MII oocytes was considered to be responsible for the compromised pregnancy outcomes [28].

Improper maturation of the oocyte cytoplasm is also believed to be one of the leading causes of polyspermy fertilization (penetration of more than one sperm) and 3PN/MPN zygotes [4,8,36], which are commonly observed in conventional IVF cycles. It remains controversial whether 3PN/MPN zygotes result from cytoplasmic immature oocytes [39,40] or over-matured oocytes [18,34]. In IVF cycles with a high proportion of 3PN/MPN zygotes, clinicians often want to know whether the cohort of oocytes were immature or over-matured before fertilization, as the information is essential for them to optimize the stimulation protocol (e.g. the dose of gonadotrophins or the time for trigger) in subsequent cycles. However, studies on the correlation between the 3PN/MPN zygote proportion and the maturation state of the oocyte cohort in IVF patients are lacking.

If 3PN/MPN zygotes mainly occur in cytoplasmic immature oocytes, the high proportion of 3PN/MPN zygotes may suggest an immature state of the oocyte cohort, and the developmental potential of embryos from that cohort may be affected, similar to the situation with a high proportion of nucleus immature oocytes (MI and GV oocytes). If 3PN/MPN zygotes occur in over-matured oocytes, we may speculate that oocytes retrieved from the fastest-growing follicles are over-matured, resulting in 3PN/MPN zygotes after insemination, while the oocytes retrieved from the second fast-growing follicles are fully matured, normally fertilized and have a high developmental potential.

Previous studies showed contradictory results of the association between high proportion of 3PN/MPN zygotes (>20% or 25%) and the pregnancy outcomes. Li observed an adverse effect of the high 3PN/MPN zygote rate on the pregnancy outcome [20], [21], [22], [23]. Dayal reported that the proportion of 3PN/MPN zygotes in the ICSI cycles was negatively correlated with the clinical pregnancy rate [9]. Oocyte maturity was not discussed in these studies. On the contrary, Golan reported that the pregnancy rate was significantly higher in cycles with polyspermy zygotes than in those without polyspermy zygotes [18].

Therefore, the purpose of our study was to investigate the association between the proportions of MPN zygotes and the maturation state of the oocyte cohort. Our findings may help evaluation of cytoplasmic maturity from a new point of view.