OBJECTIVES: The aim of this study was to analyse influence of the fetal head position and the type of instrument used (forceps, vacuum, OdonAssist™) on perineal deformation, during simulated vaginal deliveries monitored by stereophotogrammetry. METHODS: An exploratory study was conducted using mannequins simulating vaginal births. Fifty simulated deliveries were performed with different fetal head positions and instruments: Pajot's forceps, Kiwi-vacuum, and OdonAssist™. Stereophotogrammetry measured perineal deformations called equivalent strains E RESULTS: E CONCLUSION: Results confirmed increased mechanical perineal stress for OP fetal position and for forceps assisted deliveries compared to others devices. The OdonAssist™ may offer a less invasive alternative, reducing perineal strains. Stereophotogrammetry provides valuable data on the mechanical effects of childbirth.