OBJECTIVE: The purpose of this study was to assess the effect of pregnancy and first vaginal delivery on urethral striated sphincter neuromuscular function. STUDY DESIGN: Quantitative electromyographic (EMG) interference pattern analysis of the urethral sphincter of 23 nulligravidas and 31 third trimester primigravidas allowed comparison of mean motor unit parameters before term vaginal delivery and postpartum. RESULTS: Mean electromyographic interference pattern parameters in the primigravidas were significantly lower than nulligravidas even antepartum, with decreased turns, lower amplitude, and less activity. The only significant change at 6 months' postpartum was further decline in number of turns resulting in a further decrease in turns:amplitude. All other electromyographic abnormalities persisted at 6 months' postpartum and remained abnormal compared to the nulligravidas. CONCLUSION: Urethral sphincter neuromuscular function changed significantly during pregnancy and these changes persisted postpartum. Lack of recovery 6 months' postpartum suggests a physiologic impact of pregnancy itself on future risk of urinary incontinence.