Pelvic girdle pain (PGP) related to pregnancy is a common reason to sick leave during pregnancy. Low back pain and PGP affects about 50% of women during pregnancy. Most of the women recover, however about 10% of the women still have complaints after birth. Most patients have positive effect from conservative treatment, but unfortunately some do still have much pain despite intensive conservative rehabilitation. Surgery has been tried on these women with various results. Surgical treatment is controversial and there is a lack of documentation. The investigators will operate 20 patients with arthrodesis to the sacroiliac joint and symphysis. Radiostereometric analysis (RSA) will be used to evaluate the joint movement in different part of the process.