Is macrosomia predictable, and are shoulder dystocia and birth trauma preventable?

To assess our ability to detect macrosomic fetuses, and to examine the relationship between prenatal diagnosis of macrosomia and the incidence of shoulder dystocia and birth trauma.

We instituted a protocol for routine detection of macrosomic fetuses, defined as weight estimated to be at least 4500 g. Fetal weight was estimated by ultrasonography when there was clinical suspicion of macrosomia. We collected data on these pregnancies as well as on deliveries of macrosomic infants, shoulder dystocia, and birth trauma.

During the 14-month study period, there were 4480 deliveries. There were 23 macrosomic newborns (0.5%), of whom 17 were born vaginally. Six of these 17 (35%) vaginal deliveries were complicated by shoulder dystocia, and one infant sustained brachial plexus injury. The overall frequency of shoulder dystocia was 2%, the majority (93%) occurring in infants weighing less than 4500 g. Eleven newborns sustained brachial plexus injury (0.2%), and 39 had isolated clavicular fracture. Six of nine cephalic deliveries that resulted in brachial plexus injury were associated with shoulder dystocia. The sensitivity and predictive value of the study protocol were 17% (four of 23) and 36% (four of 11), respectively. Surprisingly, clinical estimation alone had a sensitivity of 43% (ten of 23) and a positive predictive value of 53% (ten of 19).

The ability to predict macrosomia is limited. The predictive value of clinical estimation of fetal weight alone may be slightly higher than when it is combined with ultrasonography. Because most cases of shoulder dystocia and birth trauma occur in nonmacrosomic infants, these conditions are practically impossible to prevent.