Relations between fetal head descent and cervical dilatation during individual uterine contractions in the active stage of labor.

The relationship between instantaneous changes in fetal head station and cervical dilatation within the individual contraction during the active stage of labor were studied and an index of labor progress was suggested. Cervix dilatation and fetal head station were measured continuously in 30 nullipara women (mean age 27.5, standard deviation 4.8). The continuous measurements enabled the analysis of each variable and the analysis of the relations between these two variables. The relationship between the head station and the cervical dilatation were demonstrated by plotting one against the other during a contraction. This led to the definition of a contraction vector that integrates the interaction between the two variables. The angle of this vector, that indicates this relation, was plotted against mean head station to demonstrate change along the delivery process regardless of time to normalize the progress and allow comparison between different women with different labor durations. This plot showed a sharp change from almost zero into a steep curve at about zero head station. A zero angle indicates that the cervix dilates during a contraction with little effect on head station while a steep angle indicates a significant effect of cervical dilatation on head station during the contraction. The contraction-vector angle reflects the changing intra-contraction relationship between head station and cervical dilatation. The angle of this vector may serve as an indicator of labor progress.