Data from three Swedish health care registries--the Medical Birth Registry, the Registry of Congenital Malformations, and the Hospital Discharge Registry--were linked for the 9-year period 1973-1981 to identify women who had appendectomy during pregnancy and their offspring, and to determine several pregnancy outcomes (gestational duration, birth weight, perinatal mortality, and congenital anomalies). Among the 720,000 deliveries during this period, 778 were complicated by appendectomy (one in 936), and the diagnosis of appendicitis was confirmed in 64% of the cases (one in 1440). Significant findings included: 1) an increase in the risk of delivery the week after appendectomy when the operation was performed after 23 weeks' gestation, with no further increase if the pregnancy continued beyond 1 week; 2) a decrease in mean birth weight of 78 +/- 24 g with more infants than expected weighing less than 3000 g; 3) an increase in the number of live-born infants dying within 7 days of birth; 4) no increase in the number of stillborn infants; and 5) no increase in the number of congenitally malformed infants.