Abstract: Objective: To study the association between the volume size of single follicular and IVF-ET/ICSI outcomes, in which the ultrasound was used to observe the single follicular developmental state in natural cycles and minimal stimulation cycles. Methods：Clinical data of 1 480 women undergoing IVF-ET/ICSI in our center between April 2007 and May 2012 were retrospectively analyzed. They were divided into 5 groups according to the follicular volume size：group A，≥5.5 mL；group B，4.5-5.0 mL，group C，3.5-4.0 mL，group D，2.5-3.0 mL，group E，1.0-2.0 mL. The clinical outcomes were compared. Results：①Follicular volume was significantly decreased with the mean diameter of single follicle on the day of hCG administration in 5 groups, in both non-stimulation protocol and minimal stimulation protocol（P＜0.001）. ②In natural cycles，the pregnancy rate and live birth rate were relatively low in group A（11.8%），while those rates in other four groups were 20.0%-25.0% and 15.8%-20.0%，but there were not significant differences in groups（P＞0.05）. In group A，the available transplantation embryo rate was high while the non-fertilization rate was low; the zero oocyte retrieval rate（32.6%） was higher than those in group C and group D（P＜0.01）; the available transplantation embryo cycle was merely 39.53%（17/43）. In group B，the 2PN rate was high while the abnormal fertilization and the not-done fertilization rate were low; the non-fertilization rate was relative high while the available transplantation embryo cycle was merely 48.19%（40/83）. The results of other C groups were better than group A generally. ③In minimal stimulation cycles，the pregnancy rates were high in group B and C（24.1% and 21.6%），but there were not significant differences when compared with three other groups（16.7%-17.8%，P＞0.05）. There were significant differences in the rates of fertilization，abnormal fertilization，good embryo and available transplantation embryo among five groups （P＜0.05）. The abnormal fertilization rate in group B was relatively high（15.4%） and the available transplantation embryo rate and the good embryo rate were highest（97.0% and 87.8%，respectively）. The good embryo rate and the available transplantation embryo rate in group E were lowest（58.1% and 75.8%，respectively，P＜0.01）. The fertilization rate in group C was lowest and the abnormal fertilization rate was also lowest（3.0%，P＜0.01）. Conclusions：IVF-ET/ICSI outcomes may be better when the volume of single follicle was 2.5-5.0 mL（15.1-19.6 mm of oocyte diameter）. However, it is necessary to have more well-designed and multicenter studies.

Key words: Fertilization in vitro, Embryo transfer, Ovarian follicle, Pregnancy outcome