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By John C. Hobbins, MD
Ginsberg and moisidis performed a retrospective analysis on 39,681 patients delivered at Northwestern University Medical Center between 1993 and 1999. Since they were interested in the recurrence of shoulder dystocia (SD), they identified patients having SD in a previous "index" pregnancy and followed these patients through subsequent pregnancies to determine what clinical clues were exhibited in those who had SD again.1
Six hundred two patients were diagnosed with SD (1.5%). Of these, 66 had a subsequent vaginal delivery at Northwestern University Medical Center. Eight had a cesarean section and 11 had a recurrence of SD. The only factors that were significantly associated with a recurrence were:
• Nulliparity during the first SD (9/11 vs 28/55 without SD);
• A higher mean infant weight (3885 g vs 3702 g; P = .03).
Perhaps the most impressive finding in the study was the rate of SD (16%) among those who had a previous delivery complicated by this problem.
SD remains one of the most frightening experiences to which a caregiver and patient can be exposed. The literature is replete with attempts to identify features of SD that would serve as warning signals to clinicians to cease and desist before or during labor. Unfortunately, many of the studies have confused the picture more than clarifying it. For example, the incidence of the complication varies according to the subjective impression of the operator. Clearly there are SDs that represent an easily solved annoyance and there are those that not only tweak the clinician’s coronaries, but can cause significant morbidity to mother and child.
If one were to define SD as the need to use maneuvers other than downward traction of the head and the McRoberts maneuver, this seems to occur in about 1-2% of pregnancies (1.5% in the above study). Although brachial plexus injuries occur in almost 15% of SD, fortunately only a small percentage persists beyond 6 months. Unfortunately, a good proportion of these end up in the courts.
The strongest clinical correlations found with SD are:
• post-term pregnancy; and
• prolonged second stage.
Even though there is a statistical association between the above factors and SD, at least half of the SDs occur in fetuses that are not macrosomic and whose mothers are not diabetic; 50% of infants sail through the second stage of labor only to surprise the practitioner with their rendition of the dreaded "turtle sign."
On the other hand, the literature does contain some very useful messages that can be put into use clinically. The one common thread that can be applied to all 4 of the above factors is body-to-head disproportion in the fetus. Many macrosomic fetuses have body-to-head disproportion even in the absence of gestational diabetes. In diabetes, glucose intolerance leads to increased fetal insulin production, as well as newly discovered fetal growth factors, which stimulate growth of the body out of the proportion to the head. Post-term pregnancy, a nearly extinct entity, is also associated with body-to-head disproportion because the trend for the abdominal circumference to increase more than the head circumference after 34 weeks of gestation continues to widen past 40 weeks. In any case, the conduct of labor is determined more by the size of the head than the body and shoulders. Simplistically, SD occurs where there is a mismatch between the shoulders and the size of the pelvis they are to pass through. When cross-sectional diameters of the head are about the same as those of the after-coming body, if the head makes it through the pelvis, it is likely the rest of the fetus will also pass through without major difficulty. In some types of macrosomia, both the head and the body are proportionately large. If these fetuses are genetically predisposed to be large, their mothers’ pelves (representing the other part of the dystocia equation) are often large enough to allow a proportionately large baby to be delivered vaginally without difficulty.
The major problem occurs when the head of a baby with body-to-head disproportion gets to a seemingly deliverable station. When the clinician is armed with this information about the presence or absence of body-to-head disproportion, he/she can make an informed choice whether or not to use forceps or vacuum and/or to set a limit on the length of the second stage of labor.
How Accurate is the Estimation of Fetal Weight?
The estimation of fetal weight (EFW) through ultrasound is definitely not a precise method. Although it gives the practitioner a ballpark idea of fetal size, the technique has been impugned in the literature, especially in macrosomia. There are more than 40 different formulas in the literature for EFW, but the ones that the practitioner uses the most (and are in the software of most ultrasound machines) employ the biparietal diameter (BPD) and/or head circumference (HC), abdominal circumference (AC), and the femur length (FL). This biometric combination will yield a volume estimate, which automatically is converted to an index of mass — the EFW.
Most formulas have a standard error of the method plus or minus 10%. Put another way, 80% of the time the estimate is within 10% of the fetal weight. This would mean that in 80% of patients if the EFW was 4000 g, the fetus is not more than 4400 g or less than 3600 g (a splay of 1½ lbs.). Twenty percent of the time the estimate is even further off.
With that in mind, rather than taking the "glass is half empty" approach, why not focus on the "glass is half full" tack? Fifty percent of the time the EFW is within 5% of the true weight and if a patient has an EFW of 4200 g, there is a 50% chance the fetus does not weigh less than 4000 g, and an 80% chance the fetus does not weigh less than 3800 g. Chances are this is a macrosomic fetus.
Once a fetus is judged to be very large, especially if the maternal pelvis is not capacious, then the presence of body-to-head disproportion becomes important.
How Can Body-to-Head Disproportion be Assessed?
Many years ago Elliott used a formula based on average diameter of the thorax at the level of the diaphragm to reflect body size.2 His thrust was to indirectly predict macrosomia by subtracting the BPD from this diameter. If the value exceeded 1.5 cm, more than 90% of the fetuses were macrosomia.
Concentrating on predictors of SD, Winn and colleagues attempted to correlate various biometric measurements of the fetus before birth with the bi-acromial diameter of the same infant in the nursery.3 Not surprisingly, the best correlation was with the clavicular length, but this was impossible to image adequately in the majority of term fetuses. The next best was the thoracic circumference at the level of the 4-chamber view of the heart. The abdominal circumference came in third.
Demonstrating that there may be more to SD than the diameter of the shoulders, Cohen and associates showed that a standard AC at the level of the umbilical vein, used in conjunction with the BPD, could be used successfully to predict SD.4 In diabetics whose EFW was more than 3800 g, if the AC minus the BPD exceeded 2.5 cm, there was a 30% incidence of SD. When the value was 2.4 cm or less, there were no cases of SD. Although the patient numbers were small in this initial report, I have been told that expanded data have born out the original findings. We have used the above formula for the last 3 years and have found it to be extremely useful.
Back to the Ginsberg and Moisidis article, where it was shown that among the factors mentioned above predisposing toward SD, having a previous pregnancy with SD ranks at the top of the list, especially if it occurred in a first pregnancy. However, rather than to empirically perform a cesarean section in patients with these risk factors, why not use all of the available tools wisely to accomplish a vaginal delivery in the overwhelming majority who will not have a SD? For example, every patient with a risk factor would have a clinical estimate of the adequacy of the maternal pelvis by an experienced examiner or by CT pelvimetry (some might consider this heresy). Also, an estimated fetal weight would be accomplished with ultrasound, as well as an estimate of body-to-head disproportion. Based on this information, the clinician would be able to choose whom to section outright (representing a small percentage of patients), whom to allow to labor on a short leash with no help from a vacuum or forceps, and whom to apply liberal expectations of progress in labor.
A 5-point scoring system could also be developed based on:
• degree of body-to-head disproportion;
• previous SD;
• SD in first pregnancy;
• estimate of adequacy of the pelvis.
Five points would either predicate a cesarean section or a low tolerance for progress in labor, and 1 or 2 points would dictate a very liberal management of progress and perhaps a gentle pull with the vacuum, if needed. One could even further refine the prelabor assessment according to the severity of the previous SD.
Obviously I am obsessing, but a prospective study, using a combination of the above factors on a large patient sampling might answer the question of how to diminish a dilemma that has plagued patients and practitioners forever.
1. Ginsberg NA, Moisidis C. Am J Obstet Gynecol. 2001;184:1427-1430.
2. Elliott JP, et al. Obstet Gynecol. 1982;60(2):159-162.
3. Winn HN, et al. J Perinat Med. 1997;25:484-487.
4. Cohen B, et al. Obstet Gynecol. 1996;88:10-13.