Protractions and Arrests of Descent

Once the cervix is fully dilated, the baby goes through an elaborate dance, called the “mechanisms of labor”, to negotiate and descend through the mother’s pelvic bones. Usually, this process of descent takes 30 minutes to 2 hours. During this period, most women feel a tremendous urge to push the baby out, and do just that.

This is the second stage of labor, and is usually the most exciting and the most tiring. Occasionally, the fetus will not “come down”. Descent problems may be problems of the power, the passenger and/or the passage. It is an exhausting problem, because until it is evident the fetus will not descend, the most effective option is for the mother to “push”. This is a tremendous expense of maternal energy that is called for at the end of an exhausting period of labor. It’s like asking someone to perform their hardest and best work in the overtime period.

Protractions (slowing down) and arrests (stoppage) of descent are often indications of malpositions, the most common of which is the occiput posterior position, OP. In OP, the baby’s face is turned toward the mother’s abdomen…..face up, instead of face down. Most babies in the OP position will turn 180-degrees and present face down at birth. This process takes a bit longer and accounts for the protraction.

Another infamous malposition is the transverse arrest. In this position the baby stays in the original position in which it engaged into the pelvis…..the transverse position, facing to the mother’s right or left. Often these babies’ heads are “deflexed” or “extended”. Instead of the chin tucked tight down on the chest, the head is tilted backwards. This causes a larger diameter of the head to present.

Arrest of descent may also be due to a head that is simply too large to fit through the mother’s pelvis…cephalo-pelvic disproportion. Sometimes, these babies will present with some degree of asynclitism. Asynclitism allows a oval-shaped object whose largest diameter is bigger than the largest diameter of the hole to fit through the hole. This is accomplished by turning the oval so that one of the prominences goes through first. Babies do this with there heads when the biparietal diameter is greater then the passageway through the mother’s pelvis.