After shoulder dystocia 187
After shoulder dystocia 187
ically determined misalignment of otherwise compatible fetal shoulder and maternal pelvic dimensions. This latter situa- tion is probably associated with a lower recurrence risk com- pared with shoulder dystocias from other causes. For these women, a recurrence may best be prevented by avoiding instrumented deliveries in the future or by patiently allowing the fetal head to spontaneously rotate to the oblique diame- ter, before traction is applied to the head and neck.
More severe shoulder dystocia occurs when the anterior shoulder becomes tightly impacted behind the pubic sym- physis. In the most severe cases, the posterior shoulder is simultaneously impacted on the sacral promontory. The most common cause for this type of shoulder dystocia is a large-for-gestational age or macrosomic fetus, whose shoul-
der width is typically 14 cm or more. 23 Less commonly, the
maternal pelvis may be contracted or have a greater trans- verse dimension at the pelvic outlet than the obstetric conju- gate, as in the platypelloid pelvis. Acquired deformities of the pelvis, such as coccygeal fracture from either trauma or a previous delivery, may also cause such difficulty in delivery of the shoulders. It is these types of shoulder dystocia events—those encountered in significantly size-discrepant mother–infant dyads—that are associated more often with
untoward outcome. 24 It is also these types of shoulder dysto- cia that would be more likely to recur in subsequent deliver- ies. The approach to such women in the next pregnancy would focus on careful monitoring of fetal growth antenatally and timely delivery for some; others with narrow or reduced pelvic capacity may be appropriately managed with planned cesarean birth. We return to these strategies later in this re- view.
The apparent simplicity of the above distinction between positionally determined and size-determined pathogenesis of the index shoulder dystocia is misleading. The contributions of positional misalignment and true fetopelvic disproportion to the occurrence of shoulder dystocia at a given delivery in a given woman are variable and are by no means mutually exclusive. A grand multipara who experiences shoulder dys- tocia after a precipitous second stage labor and delivery of an infant comparably sized to those she has delivered several times before without incident may have been caused by an acquired pelvic deformity that prevented normal occupancy of the hollow of the sacrum by the infant’s posterior shoulder. Thus, an apparent dynamically determined positional mis- alignment was also impacted on by narrowed pelvic dimen- sions. Similarly, an unanticipated finding of a randomized controlled trial of forceps versus vacuum delivery was that shoulder dystocia followed the vacuum deliveries more often
than it followed forceps deliveries. 25 One explanation for this
outcome was that the vacuum may be more apt than forceps to be successfully applied to the head of a macrosomic infant already maximally occupying the pelvic cavity since, unlike with forceps, the vacuum instrument itself does not have to
be accommodated between the head and the pelvic sidewalls. Thus, shoulder dystocias seemingly attributable to insuffi- cient time for shoulder rotation following instrumented as- sistance with delivery of the head often also involve true size
discrepancy between a large-for-gestational-age fetus and normal maternal pelvic capacity.