Oocyte donation (OD) is defined as the assisted reproduction technique (ART) in which a different woman than the one that will receive the resulting embryo provides the female gamete. Its diffusion has grown progressively due to both its excellent results and the increase of its indications [1]. According to the last European registers by the European Society of Human Reproduction and Embryology (ESHRE) in 2011, about half of the cycles of OD reported had been performed in Spain [2].

Although the impact of high serum P levels the day of triggering on IVF cycle outcome has been extensively reported, its determination is not fully implemented in daily practice. Indeed, some authors argue that this is not justified. In this text you´ll find a different approach to the analysis of the causal-effect relation of serum P and cycle outcome, to demonstrate that P levels should be measured on the day of hCG administration, when a fresh embryo transfer is planned. The 9 Bradford–Hill criteria described to analyze the potential cause–effect relation between two events are reviewed, and moreover, shown to be accomplished by the relationship between high serum P levels and pregnancy rates. Therefore, it is concluded that a simple low-cost practice as serum P determination leads to significant improvement of cycle outcome, and consequently both patients’ and practitioners’ satisfaction.