During the second half of our century, revolutionary progress has been made in obstetrics in terms of care, prevention and technology, but curiously a number of doctrinal questions have been left unresolved which are more important in cultural and practical terms than is commonly thought. We refer in particular to the physiology of the mechanics of labour whose inveterate lacunae of interpretation and major conceptual inaccuracies would be easily resolved if tackled with sufficient interest. These are easy questions to resolve because the missing or incorrect aspects of the traditional model used to interpret the mechanics of labour are for the most part only caused by clashes with the basic principles of physical science. If the mechanics of labour are re-examined with a view to anchoring them rigorously and satisfactorily to the principles of physics, the traditional model of interpretation, which is currently in vogue, is completely invalidated and replaced by another based on a series of hydrodynamic phenomena of considerable obstetric interest. The advantages of these hydrodynamic phenomena are represented by the onset of the so-called "pelvic press", doubling the motor forces of labour during the release of the fetus, the constant maintenance of a balance of forces in the fetal environment, the rotary movements caused autonomously by a hydraulic and muscular system of fixing the womb during the release of the fetus, and above all by the equal interaction of the fetus in the mechanics of its own birth. Clearly, this new interpretative model calls for the use of partly updated and partly innovative obstetric terminology.