Implementing the ESHRE 'poor responder' criteria in research studies: methodological implications.

The Bologna criteria for defining poor ovarian response (POR) during IVF provide a useful template for new research in this field of assisted conception. However, designing studies around the European Society for Human Reproduction and Embryology POR criteria can be methodologically challenging, as the new definition includes various POR subpopulations with diverse baseline characteristics and unknown clinical prognosis. When designing RCTs, potential result bias may be introduced if women from each subpopulation are not evenly allocated between intervention groups. In the case of small or moderate-size RCTs, a single-sequence randomization method may not ensure balanced allocation between groups. Stratified randomization methods provide an alternative methodological approach. Depending on the chosen methodology, patient characteristics and outcomes within each intervention group may be better reported according to relevant subpopulations.