Objective To investigate the impact of the proportion of ova obtained from large and medium follicles on day of oocyte retrieval on outcomes of in vitro fertilisation (IVF).

Methods A total of 120 infertile patients undergoing IVF treatment were selected as study subjects. Single follicle aspiration was performed for oocyte retrieval, and the volume of follicular fluid was measured. Follicular fluid diameter was converted using V=4/3πR³, dividing follicles into large (diameter≥18 mm, V≥3 mL), medium (diameter of >14 mm to < 18 mm, V of >1.5 mL to < 3 mL) and small (diameter ≤14 mm, V≤1.5 mL) categories. The proportion of oocytes derived from large and medium follicles out of the total retrieved oocytes was calculated and categorised into high-proportion group (proportion of oocytes from large and medium follicles≥71%, n=63) and low-proportion group (proportion < 71%, n=57). Comparisons were made between the two groups regarding follicular maturity, normal fertilisation rate presence of two pronuclei (2PN), fertilisation rate, embryo development and pregnancy outcomes.

Results The number of follicles with a diameter > 14 mm on the day of human chorionic gonadotropin (HCG) injection was significantly higher in the high-proportion group than that in the low-proportion group (P < 0.05). Rates of mature oocytes (MII), normal fertilisation, fertilisation, high-quality embryo, available blastocyst, frozen embryos and oocyte utilisation rates were significantly higher in the high-proportion group compared to the low-proportion group (P < 0.05). There was no significant difference between the two groups in terms of female age, the number of transferred embryos, rate of transferring high-quality embryos and embryo implantation rate (P>0.05).

Conclusion The actual size of follicles on the day of oocyte retrieval is significantly correlated with embryo quality and developmental potential. When the proportion of oocytes from large and medium follicles is 71% and above, their maturation rate, fertilisation rate, and embryo developmental potential are significantly improved.