OBJECTIVE: To determine whether there is an association between pregnancy-related back pain and changes in ligament laxity and/or relaxin. METHODS: Twenty-four women participated in three groups: normal pregnancy, IVF pregnancy, and non-pregnant controls in this prospective longitudinal cohort study, back pain was measured using a daily journal (SF-MPQ) between 8 and 36 weeks of pregnancy, and over a four week period in the IVF pregnancy and control groups. Ligament laxity was measured using a KT 2000 knee arthometer, and urine samples were collected for relaxin at 4 week intervals from the time of inclusion. RESULTS: Back pain was reported by 75% of the normal pregnancy group, 50% of IVF pregnant women, and 17% of the controls. Ninety percent of the time when laxity values increased, pain scores also increased. Relaxin results could not be obtained due to assay insufficiencies. CONCLUSION: Results indicate there is an early onset of both back pain and change in ligament laxity during pregnancy. It remains unclear whether back pain is associated with knee ligamant laxity during pregnancy