Abstract
In earlier investigations the author demonstrated that the head to cervix pressure both during and between contractions in labour is greatest at the equator of the foetal head and that the head to cervix pressure decreases towards the lower pole of the foetal head. These pressure conditions explain the biomechanics of cervical dilatation during labour under different conditions. The author has used small flat strain gauge receptors sited between the foetal head and the uterine wall. In the present investigation the head to cervix pressure was also measured above the equator of the foetal head. Between the contractions the head to cervix pressure decreases progressively above the equator of the head and finally the amniotic fluid pressure is measured. In some cases, especially primi-gravidae, the head to cervix pressure curves rise very steeply at the beginning of a contraction but as the amniotic pressure approaches its maximum, the head to cervix pressure decreases and ultimately falls to the same level as the amniotic fluid pressure. In other women, especially multigravidae, the increase of the head to cervix pressure at the beginning of the contraction is less, but usually the pressure at the beginning of the contraction increases more rapidly than the head to cervix pressure below the equator. During the contractions the amniotic pressure was measured first at high level and at higher amniotic pressure successively towards the equator. The findings are supported by model experiments using a wooden sphere over which a rubber sheath has been drawn. The investigations explain the engagement of the foetal head in the lower part of the uterus. The importance of the engagement of the foetal head when the vertex presents is related to the influence upon the friction between the foetal head and the uterine wall and the mechanism of cervical dilatation. The engaged foetal head prevents the outflow of amniotic fluid after rupture of the membranes and minimises the risk of prolapse of the umbilical cord.