Increased birth weight is associated with more difficult delivery and increased rates of neonatal injury. A number of studies have used shoulder dystocia and initial injury rates as the outcome measures. The association between method of delivery and birth injury has been studied, but conclusions have varied. Kolderup and colleagues examined the association between the delivery method and persistent birth injury in infants with birth weights of 4,000 g (8 lb, 13 oz) or greater.

A total of 2,924 macrosomic infants were included in the study. The outcomes in these infants were compared with outcomes in 16,711 infants with birth weights from 3,000 g (6 lb, 10 oz) up to 4,000 g (8 lb, 13 oz).

A total of 48 injuries were diagnosed in the macrosomic cohort, for an overall injury rate of one injury per 61 deliveries (1.6 percent). Injuries ranged from simple clavicular fractures to multiple nerve palsies. One infant in the macrosomic group died following a 30-minute dystocia. In contrast, the injury rate in the nonmacrosomic infants was one injury per 288 deliveries (0.35 percent).

The injuries sustained in the macrosomic group tended to be more severe and were more likely to persist. A total of 22 brachial plexus injuries occurred, and all five that persisted for as long as six months after birth occurred in macrosomic infants. The data indicated that macrosomia was associated with an increased risk of persistent injury (relative risk: 6.7), and the risk was strongest for forceps-assisted and spontaneous vaginal deliveries. Cesarean section was associated with minimal risk of injury in both study groups.

Delivery by forceps (low or outlet) correlated with an approximately fourfold greater risk of persistent clinical findings at age six months, compared with vaginal delivery or cesarean section in the infants weighing more than 4,000 g (8 lb, 13 oz). Vacuum delivery resulted in the same rates of injury, but the injuries had resolved by the six-month follow-up visit.

Analysis of the subgroup of macrosomic infants born to mothers with diabetes revealed that the cesarean section rate in this group approached 70 percent. Severe birth injuries occurred in three of 78 infants delivered vaginally.

The authors used their findings to calculate the number of additional cesarean sections that would have been required to prevent birth injury in all of the neonates in the macrosomic group. The data indicated that if all macrosomic infants could be identified antenatally, 258 additional cesarean sections would be needed to prevent one persistent injury.

The authors conclude that the outcomes in the infants in this study suggest that a trial of labor is recommended even for macrosomic infants. Although persistent injury is more common in macrosomic infants delivered by forceps, the rate of documented lasting sequelae is very low. This low rate of persistent injury, combined with the current limitations of estimating the weight of large fetuses, supports the careful and judicious use of operative vaginal delivery for infants with suspected macrosomia.