It is necessary to know something about the anatomy of the fetus and the maternal pelvis in order to understand how shoulder dystocia comes about and how it causes the injuries it does.

As the accompanying diagram shows, the maternal pelvis is composed of a series of bones forming a circle protecting the pelvic organs. The front-most bone is the symphysis pubis. It is on this structure that a baby's anterior shoulder gets caught during a delivery complicated by shoulder dystocia. The bone at the back of the maternal pelvis is the sacrum. Because of its shape, it generally serves as a slide over which a baby's posterior shoulder can descend freely during labor and delivery. The side walls of the maternal pelvis, although very important in determining the ease of the process of labor in general, usually do not contribute to shoulder dystocia.

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In normal vaginal deliveries the head of the baby, called the "vertex", emerges first. During labor, the soft, mobile bones of the fetal head can "mold"-alter their shape -- and, to a slight degree, overlap. This facilitates the fetal head fitting into and through the maternal pelvis. The baby's shoulders, likewise being flexible, usually follow the delivery of the baby's head quickly and easily. But for this to happen, the axis of the fetal shoulders must descend into the maternal pelvis at an angle oblique to the pelvis's anterior-posterior dimension. This position affords the shoulders the most room for their passage. If instead the shoulders line up in a straight front-to-back orientation as they are about to emerge from the mother's pelvis, there will often be insufficient room for them to squeeze through. The back of the mother's pubic bone then forms a shelf on which the baby's anterior shoulder can get caught. If this happens, the shoulders cannot deliver and a shoulder dystocia results.

Shoulder dystocia can also occur if the posterior shoulder of a baby gets caught on its mother's sacrum. This is a far less common cause of shoulder dystocia. The sacrum, having no protrusions equivalent to that of the pubic bone, is far less likely to impede the descent of the baby's posterior shoulder.

As can be readily appreciated, it is the relative sizes of the fetal head, shoulders, and chest compared to the shape and size of the maternal pelvis that determine how smoothly a delivery will go. Usually it is the fetal head that has the largest dimensions. Thus if it can pass through the maternal pelvis without difficulty, the rest of the baby usually follows easily. However, when the dimensions of the fetal shoulders or chest rival those of its head, the chances of a shoulder dystocia occurring are much increased. Such situations occur more frequently both in large babies and in babies of diabetic mothers.

In large babies, much of the excess growth that occurs is in the chest and abdominal areas. In these babies the dimensions of the shoulders and chest tend to be disproportionately larger than those of the head. This trend is exaggerated in babies of diabetic mothers. Multiple studies have shown that babies of diabetic mothers more frequently have larger ratios of shoulder circumference to head circumference than do their peers born of nondiabetic mothers. Babies of diabetic mothers also have greater arm circumference, larger triceps folds, and a higher percentage of body fat. Since larger babies are more likely to "get stuck", much of the work in the field of shoulder dystocia has been targeted at attempting to predict which babies will be larger than normal, especially when their mothers are diabetic.

Except in extraordinary circumstances, once the fetal head and shoulders have been delivered the remainder of the fetal trunk and legs slide out easily. Such extraordinary circumstances preventing easy delivery of the fetal body might be when:

• A fetus has a large abdominal or lower back tumor,
• The umbilical cord is wrapped tightly around the baby's neck, or
• There is a severe constriction of the uterine muscle -- "contraction rings" -- trapping the baby in the uterus.

The above applies only to vertex or headfirst deliveries. Breech deliveries, where the fetal legs and buttocks emerge first from the vagina, can also result in injury to the fetal arms and neck, producing the brachial plexus injuries discussed above. However, since these and other sorts of injuries to babies from vaginal breech deliveries occur at a relatively high rate, most breech babies in the United States are now delivered by cesarean section.

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