Pressures on the fetal head during normal labor.

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.