In the present analysis, while oocyte recovery rate was non-significant decrease in follicles ≥ 24 mm, the ratios of 2PN and TQE per follicle were comparable. Moreover, zygotes derived from large follicle (≥ 24 mm) yielded a non-significantly higher TQE rate.
The association between follicular size and oocyte maturity has been studied already three decades ago and it dictated the timing of final follicular maturation trigger, whenever several follicles reach diameter of > 17–20 mm (6–7, 10–11). In our previous study (8), we demonstrated higher oocyte recovery rate in large (≥ 16 mm) and medium (13–15 mm) large compared to the small (< 13 mm) follicle groups, finding which is consistent with previous studies (2, 11, 12, 13). Moreover, MII oocytes were more commonly found in the medium and large follicle groups (8, 11, 14).
In a prospective study conducted by Triwitayakorn et al. (13), including 991 follicles, fertilization rate of mature oocytes, as well as the rate of good quality embryos showed a tendency to increase from the small follicle group to the large follicle group, however, this finding was not significant. Dubey et al. (7) reported that oocytes fertilization rate had a positive linear correlation as follicle diameter increased, while Nogueira et al.(15) demonstrated that matured oocytes retrieved from small follicles generated embryos of lower developmental potential than oocytes derived from larger follicles.
In the majority of studies, large follicles relate to those ≥ 16 mm in diameter. Only few considered larger (≥ 24 mm) diameters. In the present analysis we observed a non-significant decrease in oocyte recovery rate in follicles ≥ 24 mm, with comparable 2PN and TQE per follicle ratios. However, once a zygote was recovered from large follicle (≥ 24 mm), a non-significantly higher TQE rate was observed. In accordance with our observation, Ectors et al. (6) observed that follicles of > 23 mm on the day of oocyte retrieval had higher maturation rate compared to those < 23 mm. Moreover, the % of good scored embryos was demonstrated to increase from 55.4% of follicles size of 16–23 mm, to 64.6% of follicle > 23 mm (Ectors et al. (6).
In most centers, final follicular maturation is triggered once two to three follicles reach at least 17–18 mm in diameter, actually 2 days prior to oocyte retrieval. Sometimes, few follicles reach the required size while others are still small or medium size, and it is common to "scarify" the larger, on behalf of allowing the development of the smaller cohort of follicles. In the present study we could demonstrate, that by letting follicles to develop to large diameter (≥ 24 mm), not only that they are not scarified, but they have good probability to yield MII oocytes, and once recovered, to develop to TQEs.
Strengths and Limitations
The study has several limitations. Women included in the study were treated for infertility due to various reasons. Furthermore, treatment protocols were not homogeneous to all study population, therefore, follicles exposed to different gonadotropins and trigger modes were included. This might have influenced the chance to achieve oocyte during retrieval.
Although various studies exist concerning the association between follicular size to oocyte recovery rate at retrieval, data relating to large (≥ 24 mm) follicles is scarce. This study strength is in its being conducted in a single center by professional consistent team on a large study group.