Physiology of Normal Labor and Delivery

Objectives

1. To understand and recognize a normal labor pattern.
2. To understand the mechanism of labor for a cephalic presentation.
3. To understand the meaning of the following germs: Presentation, position, lie, station, effacement, dilatation.
4. To understand the phases and stages of labor.

Physiology of Normal Labor and Delivery

1. Normal labor
   
   Emanuel Friedman in his elegant treatise on labor (1978) stated correctly that "the clinical features of uterine contractions namely frequency, intensity, and duration cannot be relied upon as measures of progression in labor nor as indices of normality. Except for cervical dilatation and fetal decent, none of the clinical features of the parturient patient appears to be useful in assessing labor progression." Friedman sought to select criteria that would limit normal labor and thus be able to identify significant abnormalities of labor. These limits, admittedly arbitrary, appear to be logical and clinically useful. The graphic representation of labor plotting descent and dilatation against time has become known as the Friedman curve. It, or a modification of it, is used extensively to evaluate laboring patients.
   
   Figure 2. Graphic portrayal of the relationship between cervical dilatation and elapsed time in labor (heavy line) and between fetal station and time (light line). Labor has been divided functionally into a preparatory division (including latent and acceleration phases of the dilatation curve), a dilatational division comprising only the linear phase of maximum slope of dilatation, and a pelvic division encompassing the linear phase of maximum descent.