All pregnant women from a well defined area (the central district of the County of Ostergötland, Sweden) attending antenatal clinics over a period of seven months were interviewed with regard to low back pain during pregnancy. Of 862 women who answered the questionnaires, about half developed some degree of low back pain. Seventy-nine women who were unable to continue their work because of severe low back pain were referred to an orthopedic surgeon for an orthoneurologic examination. The most common reason for severe low back pain was dysfunction of the sacroiliac joints. Physically strenuous work and previous low back pain were factors associated with an increased risk of developing low back pain and sacroiliac dysfunction during pregnancy.