COMPARISON OF FERTILIZATION, CLEAVAGE AND PREGNANCY RATES OF OOCYTES FROM LARGE AND SMALL FOLLICLES

Ovarian stimulation for in-vitro fertilization (IVF) causes developmen t of several cohorts of follicles, At the time of oocyte collection, o ocytes are thus retrieved from a wide range of follicles of different sizes and developmental stages. A relationship between size of follicl es and pregnancy rates has earlier been demonstrated. The ain of the p resent study was to compare fertilization, cleavage and pregnancy rate s between oocytes retrieved from large and small follicles in conventi onal IVF and intracytoplasmic sperm injection (ICSI), A total of 200 c onventional IVF patients and 175 ICSI patients underwent oocyte retrie val where oocytes from both large and small follicles were collected. A follicle with a volume of greater than or equal to 2 ml, correspondi ng to a follicular diameter greater than or equal to 16 mm as determin ed by ultrasound, was regarded as a large follicle. Only one cycle fro m each patient was included, Fertilization and cleavage rates were cal culated per patient for oocytes from large and small follicles. The me an fertilization and cleavage rates for conventional IVF and ICSI cycl es were calculated, Comparison of pregnancy rates was performed for pa tients receiving embryos derived from oocytes of only large or only sm all follicles, For conventional IVF patients, fertilization rates were 71.4 and 58.1% (P < 0.01, Wilcoxon paired test) for oocytes of large and small follicles respectively. The corresponding cleavage rates wer e 95.4 and 93.9% respectively. The pregnancy rate for the two groups w as 47% (60/127) and 15% (2/13) (P < 0.05, chi(2) test). For ICSI patie nts the fertilization rate was 72.0 and 71.1% for oocytes of large and small follicles respectively. The corresponding cleavage rate was 93. 0 and 91.1%. The pregnancy rate in the two groups was 41% (46/113) and 42% (5/12), The results show that oocytes from smaller follicles also yield fertilization and pregnancies, although in conventional IVF to a lesser extent than oocytes from larger follicles. For IVF cycles, a higher proportion of immature oocytes (which are normally not included in the ICSI procedure) in the group of oocytes from small follicles i s most probably the explanation for the lower fertilization rate. The decrease in pregnancy rate with oocytes from small follicles in the IV F cycles was not observed in the ICSI cycles. The possibility of evalu ating the degree of oocyte maturation prior to fertilization may be an advantage of the ICSI technique. This suggests that the disadvantages of oocytes from small follicles might be overcome by means of ICSI.