In an attempt to define the risk to the fetus associated with anesthesia and surgery during pregnancy, a study was performed using health insurance data from the province of Manitoba (1971 to 1978). Each of the 2,565 women undergoing incidental surgery during pregnancy (Group A) was paired with a pregnant female not undergoing surgery (Group B) by maternal age and area of residence. Both groups were linked to a separately maintained provincial congenital- anomalies registry to ascertain the frequency of anomalies. There was no signficant difference in the rate of congenital anomalies between the two groups, implying no strong teratogenic effect. However, there was an increased risk of spontaneous abortion in those undergoing surgery with general anesthesia in the first or second trimester, most notably after gynecologic procedures (estimated risk ratio = 2.00), but also following procedures anatomically remote from the conceptus (estimated risk ratio = 1.54). While it is concluded that surgery with general anesthesia is associated with a higher incidence of abortion, it is conjectural at present which factor(s) account for the observed increase in fetal risk.