Reproductive potential of a metaphase II oocyte retrieved after ovarian stimulation: an analysis of 23 354 ICSI cycles.

BACKGROUND Live birth per cycle and live birth per embryo transfer are commonly used outcome measures for IVF treatment. In contrast, the assessment of the reproductive efficiency of human oocytes fertilized in vitro is seldom performed on an egg-to-egg basis. This approach may however gain importance owing to the increasingly widespread use of oocyte cryopreservation, as the technique is becoming more established. The aim of the current study is to quantify the reproductive efficiency of the oocyte according to ovarian ageing and ovarian response. METHODS We performed a retrospective analysis of the outcome of all consecutive patients attending for treatment between 1992 and 2009. The outcome in terms of live birth after fresh and cryopreserved embryo transfer per mature oocyte was calculated for 207 267 oocytes retrieved in 23 354 ovarian stimulation cycles. The oocyte utilization rate (number of live births per mature oocyte) was further analysed in relation to the ovarian response. RESULTS The oocyte utilization rate in women in the age of ≤ 37 years remains constant with a mean of 4.47% live birth per mature oocyte [95% confidence interval (CI): 4.32-4.61]. From the age of 38 years onwards, a significantly lower oocyte utilization rate was noted, declining from 3.80% at the age of 38 years to 0.78% at 43 years (P < 0.001). In this 38-43 years age group, oocyte utilization rate was no longer dependent on ovarian response (P = 0.87). CONCLUSIONS The major strength of the study, which is also its weakness, is the fact that we included a large number of cycles performed over a long period of time. According to our results, the oocyte utilization rate between 23 and 37 years of age depends largely on ovarian response and to a much lesser extent on age. From the age of 38 years onwards, the utilization rate depends largely on age and to a much lesser extent on ovarian response. Considering the increased use of oocyte freezing for fertility preservation, these data are extremely valuable as they provide an estimate of the number of oocytes needed to achieve a live birth.