Abstract Objective To evaluate the influence of a maternal position for childbearing, that of lateral decubitus compared with the dorsal decubitus in second phase of parturition, by studying various parameters such as: rate of intact perineum, electronic foetal heart rate monitoring during expulsion, mode of delivery, quantity of maternal blood losses, and neonatal status. Patients and methods The type of study was as follows: prospective, comparative and randomized according to Zelen's design, monocentric, in pragmatic mode and intention to treat. After a review of the literature and a retrospective analysis of the activity of a service (Romans-sur-Isère/France), an assumption of improvement of 15% of the rates of intact perineum in lateral position is put forth. An amount of 562 patients was included and randomized in 2 groups in which the childbirth was carried out in dorsal or lateral decubitus. The data were analysed by software SPSS version 13, by using the tests t of Student, Chi 2 of Pearson or the exact test of Fisher. The analysis relates to 487 patients after exclusion of the distortions to the protocol. Results We observed a significant increase in the rate of intact perineum, as well as a significant increase in the maternal blood loss, without exceeding the physiological limits, during expulsion in lateral position. The rate of directed delivery was less but more frequent were the haemorrhages in decubitus side. The rate of artificial delivery and/or uterine revision doubled without however reaching the threshold of significativity. Finally, the other parameters did not differ between the two groups. Discussion and conclusion The test makes it possible to show the improvement of the perineal status during dorsal childbirth in decubitus side compared with the position in decubitus. The data collected prove to be in conformity with those of the literature, which is not very prolix on the subject! The secondary criteria are not improved, and professionals must carry out a reflexion concerning delivery, to adapt the recommendations for the introduction of such practice into a service, must carry out a reflexion. This study provides the scientific basis (NP1), which makes it possible to justify the diffusion of this position of expulsion if one is keen on the improvement of the perineal status.