Pressures on the fetal head during normal labor

Abstract

The pressure recordings from both parietal regions of the fetal head during the second stage of labor of 44 spontaneous deliveries with vertex presentation were evaluated and compared to the simultaneously recorded pressures in the amniotic cavity by calculating pressure parameters during and between uterine contractions. Analysis of the bearing down period revealed on the average higher values at the anterior side in comparison to the posterior side of the skull. This is explained by the complex anatomy of the curved birth canal. Overall, the head pressure values averaged from the recordings of both parietal regions were about twice as high as the corresponding amniotic pressure values. During the descend of the head through the birth canal, an increase of pressure on the fetal head was observed that is mainly due to the rising resistance of the narrowing pelvis and stretched tissue of the passageway. A marked interindividual deviation of head pressure was found that was independent from amniotic pressure variations thus preventing indirect deduction of head strain from internal togography.

Biomechanical Analyses of the Efficacy of Patterns of Maternal Effort on Second-Stage Progress

“…(10, 11) Rempen showed that the mean pressure on the fetal head in the amniotic cavity was 8.5 kPa during a uterine contraction, while a voluntary push increased the pressure to 19 kPa. The average basal tone during the rest interval was 2.6 kPa.…”

“…The average basal tone during the rest interval was 2.6 kPa. (10) As a first-order approximation, the expulsive force was simulated by applying uniform expulsive pressure to the “cranial” hemisphere of the fetal head, the equator of which was constrained to lie perpendicular to the Curve of Carus as the head progressed through the pelvic floor. The Curve of Carus was defined as a locus of points lying 6 cm dorsal to the posterior margin of the pubic symphysis (Figure 1D).…”

On the Biomechanics of Vaginal Birth and Common Sequelae

“…During this approximately 90 min stage, spontaneous uterine contractions occur every 3 min for a duration of 1 min. These contractions increase the uterine pressure near the fetal head from 2.6 to 8.5 kPa (6), engaging the head in the pelvis and helping to push it through the birth canal along what is known as the Curve of Carus. When, during the second stage of labor, the woman is instructed to push during uterine contractions by bearing down via a maximal contraction of her diaphragm, this increases the intrauterine pressure to approximately 19 kPa.…”

Quantification of friction force reduction induced by obstetric gels

“…During trials with the distilled water, an increase of 10 kPa contact pressure resulted in an increase of approximately 1.5 N friction force, whereas during trials with obstetric gel, an additional 10 kPa resulted only in an increase of approximately 0.5 N. Thus, friction forces of studies with a constant contact pressure can be extrapolated to other contact pressures of interest. In the present work, the contact pressure was set to 30 kPa simulating the pressure usually found between the human fetal head and cervix [11]. It was assumed that under this pressure, friction forces will not depend on the deformation of both the fetus' body and birth canal.…”