Dystocia is the consequence of four distinct abnormalities that may exist singly or in combination:
1. Abnormalities of the expulsive forces, either uterine forces insufficiently strong or inappropriately coordinated to efface and dilate the cervix—uterine dysfunction—or inadequate voluntary muscle effort during the second stage of labor.
2. Abnormalities of presentation, position, or development of the fetus.
3. Abnormalities of the maternal bony pelvis—that is, pelvic contraction.
4. Abnormalities of soft tissues of the reproductive tract that form an obstacle to fetal descent
The cervix and lower uterus are shown at the end of pregnancy and at the end of labor. At the end of pregnancy, the fetal head, to traverse the birth canal, must encounter a relatively thicker lower uterine segment and undilated cervix. The uterine fundus muscle is less developed and presumably less powerful. Uterine contractions, cervical resistance, and the forward pressure exerted by the leading fetal head are the factors influencing the progress of the first stage of labor.
After complete cervical dilatation, however, the mechanical relationship between the fetal head size and position and the pelvic capacity, namely fetopelvic proportion, becomes clearer as the fetus descends. The uterine musculature is much thicker and thus more powerful. Accordingly, abnormalities in fetopelvic proportions become more apparent once the second stage is reached.
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