To define the risk of adverse reproductive outcomes after nonobstetric operations during pregnancy, we linked data from three Swedish health care registries, the Medical Birth Registry, the Registry of Congenital Malformations, and the Hospital Discharge Registry, for the years 1973 to 1981. Adverse outcomes examined were the incidences of (1) congenital anomalies, (2) stillborn infants, (3) infants dead at 168 hours, and (4) infants with very low and low birth weights. There were 5405 operations in the population of 720,000 pregnant women (operation rate, 0.75%). The incidences of congenital malformations and stillbirths were not increased in the offspring of women having an operation. However, the incidences of very-low- and low-birth-weight-infants were increased; these were the result of both prematurity and intrauterine growth retardation. The incidence of infants born alive but dying within 168 hours also was increased. No specific types of anesthesia or operation were associated with increased incidences of adverse reproductive outcomes. The cause of these outcomes was not determined.