Both our previously performed decision analysis and more recent clinical data considered in the context of our decision analytic framework support the claim that in the pregnancies of women without diabetes the level of intervention and the economic costs of prophylactic cesarean delivery for fetal macrosomia diagnosed by means of ultrasonography are predicted to be excessive. Under the most plausible assumptions, a prophylactic cesarean policy with either a 4000- or 4500-g macrosomia threshold would require more than 1000 cesarean deliveries and millions of dollars to avert a single permanent brachial plexus injury. In the pregnancies of diabetic women, although such policies would be expected to perform appreciably better, their use would nevertheless entail considerable intervention for any benefit achieved. Under most assumptions, hundreds of cesarean deliveries and hundreds of thousands of dollars would be required to avert a single permanent brachial plexus injury. In light of the available data, optimizing the management of shoulder dystocia seems at present to be the most immediate and tenable approach to the prevention of birth-related brachial plexus injury. (Am J Obstet Gynecol 1999;181:332-8.)