[Obstetrical mechanics. Intermediate stage and childbirth stage].

It is possible to divide the classical second stage of labour--the stage of expulsion--into two different anatomical and functional parts. The one phase is the intermediate phase where dilatation of the cervix is almost complete and where there is an instinctive reflex that delivery is going to take place and this is very strong, and this is what starts off the oscillation (contra-nutation) of the sacrum and full engagement. Then there is the phase of the delivery itself where the sacrum must slide and rotate, and which cannot be accomplished without perineal relaxation. The utero-sacral ligaments demonstrate at the end of pregnancy and in early labour that they are the true tendons into which the back of the gravid uterus is inserted. They take part in the oscillation of the sacrum. Low backache that occurs at the end of pregnancy and in labour is due to strong tension on the utero-sacral ligaments, when it is difficult for the head to engage, and can only be relieved when the pelvis tips forward. When delivery is taking place it is important to be rid of all expulsive efforts that may cause contraction of the anterior part of the pelvis.