Shoulder Dystocia
Abstract & Commentary
By John C. Hobbins, MD, Professor of Obstetrics and Gynecology, University of Colorado School of Medicine, Aurora, CO, is Associate Editor for OB/GYN Clinical Alert.
Dr. Hobbins reports no financial relationships relevant to this field of study.
Synopsis: A recent case-control study shows that fetuses with brachycephaly have a greater risk of shoulder dystocia, and when the occipital-frontal measurement is added to an algorithm that includes maternal weight, estimated fetal weight, and history of diabetes, shoulder dystocia can be suspected or excluded with reasonable accuracy.
Source: Belfort MA, et al. Association of fetal cranial shape with shoulder dystocia. Ultrasound Obstet Gynecol 2012;39:304-309.
For years, clinicians have been trying to find ways to predict shoulder dystocia (SD), a complication that can have serious consequences for some infants and, on occasion, for the providers who delivered these babies. Although it is clear that large mothers, large babies, and diabetes are loaded decks for SD, at least half of SD occurs in women with normal BMIs and in babies who weigh less than 4000 grams. Various algorithms have been fashioned to avoid SD, which put into play various maternal and fetal characteristics. A recent report adds a new variable to the mix: the shape of the fetal head.1
The authors sifted through 335 cases of SD delivering at one hospital between 2000 and 2010 and found only 18 cases in which the estimated fetal weight (EFW) was calculated within 21 days of delivery and complete clinical and historical data were available. They identified a separate control group of 18 patients who were matched to each SD case by gestational age, maternal BMI, glucose status, EFW, maternal age, parity, and fetal information.
The authors were interested in determining if the fetal head shape independently correlated with SD. The head circumference (HC) and biparietal diameter (BPD) were established by standard procedure, and the occipital frontal diameter (OFD) was calculated retrospectively from both of these measurements. Shoulder dystocia was diagnosed by the individual operators delivering the affected babies.
Of all the variables analyzed, the BPD divided by the OFD (the cephalic index) correlated best with SD, as did a small OFD alone. The birth weight was statistically significantly larger in the SD cases, but the data on EFW were unclear. However, the head shape had more impact on SD than any other variable. So when the authors added the OFD to a formula consisting of maternal weight, presence or absence of diabetes, and EFW, there was a reasonable ability to predict SD. Using a receiver-operator curve-derived threshold, this formula had a sensitivity and specificity for SD of 86% and 95%, with a positive predictive value and negative predictive value of 16% and 99.8%, respectively. This result was substantially better than using the EFW alone (which seemed to be generally underestimated in the study).
Commentary
Why would a brachycephalic configuration predispose a fetus to SD? The authors felt that either it was easier for the head to come through the pelvis before the shoulders were ready, or that the manner in which a small OFD entered the pelvis altered the descent of the shoulders in a way that would discourage rotation. I'll buy either explanation to a degree. The reason diabetics have a higher rate of SD is because the fetal head determines the conduct of labor and infants of diabetics tend to have normal size heads but large chests, shoulders, and abdomens. Therefore, the head will come through without a problem (often without a clue from the labor curve), but then, as the shoulders become progressively wedged in, there will be a point of no return.
The other interesting finding was that the index we have been using to predict SD — the average abdominal diameter minus the BPD, first described by Cohen et al2 and validated in a larger population by Miller et al3 — was not effective in predicting shoulder dystocia in this study.
I think the biggest problem with this study is the small number of patients included. One wonders how different the results might have been if the authors had adequate information on more of the 317 patients with SD that could not be included in the study. The study certainly is interesting enough to begin assessing this algorithm in our own patients here in Colorado.
References
- Belfort MA, et al. Association of fetal cranial shape with shoulder dystocia. Ultrasound Obstet Gynecol 2012;39:304-309.
- Cohen B, et al. Sonographic prediction of shoulder dystocia in infants of diabetic mothers. Obstet Gynecol 1996;88:10-13.
- Miller RS, et al. Sonographic fetal asymmetry predicts shoulder dystocia. J Ultrasound Med 2007;16:1523-1528.
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