Effects of the individual uterine contraction on fetal head descent and cervical dilatation during the active stage of labor

Abstract Objective To evaluate the effects of individual uterine contractions on instantaneous values of cervical dilatation and head station along the active stage of labor. Study design Cervix dilatation and fetal head station were measured continuously using a labor monitor that is based on ultrasonic triangulation. The relations between the two variables in response to each contraction were analyzed. The relative effect of the contraction on head station and on cervical dilatation was demonstrated by plotting one against the other during the contraction and quantified by two indices: (a) the contraction vector that integrates the maximum effect of uterine contraction on both variables and (b) the efficiency vector that indicates the contribution of each contraction to labor progression. The amplitude and angle of each vector were calculated. Correlation between the waveforms of head station and cervix dilatation during contractions was also calculated. These indices were plotted against cervix dilatation and head station at different stages in labor progress. Results Effects of uterine contractions on cervix dilatation and head station varied during labor. The amplitude of the contraction vector and efficiency vector increased to a maximal value at cervical dilatation of 6 cm. The angle of the contraction vector increased with the progress of labor. Correlation between cervical dilatation and head station was maximal at the engagement zone of the birth canal. High variability was observed between subjects for all indices measured. Conclusion The contraction vector and the efficiency vector exhibited distinct behavior during labor. These vectors may serve as indicators for normal and abnormal progress of labor. More data are required to obtain statistical significance.