Embryo implantation rates in natural and stimulated assisted reproduction treatment cycles in poor responders

Controlled ovarian stimulation with exogenous gonadotrophins and gonadotrophin-releasing hormone (GnRH) analogues enables the collection of multiple oocytes and subsequent development of multiple embryos. However, interfering with the natural hormonal milieu may decrease the probability of successful embryo implantation due to effects on oocytes and/or endometrium. In order to provide a fair comparison of embryo implantation rates between natural cycles and stimulated cycles, bias caused by the presence of multiple embryos available for transfer in stimulated cycles should be avoided. This retrospective study analysed embryo implantation rates in cycles in which only a single embryo was available for transfer in 304 women who had poorly responded to ovarian stimulation in the previous cycle. Embryo implantation rates with different stimulation protocols were as follows: natural cycle, 20% (6/30); gonadotrophin only, 5.6% (3/54); long GnRH protocol, 3.8% (2/52); co-flare protocol, 1.9% (1/52); microdose flare-up, 15.4% (4/26); GnRH antagonists, 14.4% (13/90). Although the difference was not statistically significant there was a trend towards higher implantation rates with natural cycles in this group of women. Natural cycle IVF may be a reasonable and patient-friendly treatment choice yielding an acceptable outcome for women who are known or anticipated poor responders to ovarian stimulation.