Mathematical relationships between uterine contractions, cervical dilatation, descent and rotation in spontaneous vertex deliveries.

OBJECTIVE: To determine the mathematical relationships between the strength and duration of the uterine contractions, the descent and rotation of the fetal head and the degree of cervical dilatation in 50 multiparous women with spontaneous vaginal deliveries using a simple device applied to the fetal vertex.
METHOD: A simple device for monitoring the progress of labor was applied to the fetal vertex. The device allows the continuous monitoring of descent and rotation of the fetal head. The amount of descent and the degree of rotation were also determined by repeated vaginal examinations as well as the degree of cervical dilatation. The frequency of uterine contractions was also recorded on a partogram. RESULT: A good correlation was found between the amount of descent of the fetal vertex (r = 0.975) and between the degree of rotation of the fetal head (0.83) determined by both methods. Multiple regression analysis was then performed and the degree of cervical dilatation in cm at any given time during the first stage of labor was found to be equal to 2.859 + 0.583 fetal head station in (cm) + 0.1983 internal rotation in degrees -0.0493 (station x internal rotation) + 0.1599 station2 + 0.3622 uterine contractions per 10 min. A nomogram was constructed allowing the calculation of cervical dilatation for a given station of the head, degree of rotation and frequency of uterine contractions.
CONCLUSION: There is a defined mathematical relationship between the degree of descent and rotation of the fetal head, the degree of cervical dilatation and the frequency of uterine contractions in multiparous women with vertex presentation. The first three variables can be continuously determined by using the described device. Incorporation of the device into a reusable fetal scalp electrode allows the dual mechanical and electronic monitoring during labor with minimal vaginal examinations.