Our aim was to determine the cost-effectiveness of a policy of elective C-section for macrosomic infants to prevent maternal anal incontinence, urinary incontinence, and newborn brachial plexus injuries. We used a decision analytic model to compare the standard of care with a policy whereby all primigravid patients in the United States would undergo an ultrasound at 39 weeks gestation, followed by an elective C-section for any fetus estimated at > or =4500 g. The following clinical consequences were considered crucial to the analysis: brachial plexus injury to the newborn; maternal anal and urinary incontinence; emergency hysterectomy; hemorrhage requiring blood transfusion; and maternal mortality. Our outcome measures included (1) number of brachial plexus injuries or cases of incontinence averted, (2) incremental monetary cost per 100,000 deliveries, (3) expected quality of life of the mother and her child, and (4) "quality-adjusted life years" (QALY) associated with the two policies. For every 100,000 deliveries, the policy of elective C-section resulted in 16.6 fewer permanent brachial plexus injuries, 185.7 fewer cases of anal incontinence, and cost savings of $3,211,000. Therefore, this policy would prevent one case of anal incontinence for every 539 elective C-sections performed. The expected quality of life associated with the elective C-section policy was also greater (quality of life score 0.923 vs 0.917 on a scale from 0.0 to 1.0 and 53.6 QALY vs 53.2). A policy whereby primigravid patients in the United States have a 39 week ultrasound-estimated fetal weight followed by C-section for any fetuses > or =4500 g appears cost effective. However, the monetary costs in our analysis were sensitive to the probability estimates of urinary incontinence following C-section and vaginal delivery and the cost estimates for urinary incontinence, vaginal delivery, and C-section.