Pelvic girdle pain (PGP) during pregnancy is common and, indeed, has always been considered normal. It is commonly associated with moderate to severe pain that impairs everyday activities such as getting up from a chair, bending, walking, working in the home and caring for children, as well as, of course, paid employment. Also, PGP is a frequent cause of sick leave during pregnancy. The aetiology of PGP is poorly understood and there is no official nomenclature, no effective evidence- based preventive measures or treatment, known risk factors or detailed knowledge of the clinical course of the various subgroups of this condition. Objectives The objectives for this project were to determine the prevalence of PGP during pregnancy in a random population of women, detect factors associated with the development of this condition, explore what influences taking sick leave due to PGP, and examine whether pregnant women with PGP, who have been sub-grouped on the basis of two clinical tests, differ with regards to demographic characteristics and/or the clinical course of PGP during the second half of their pregnancy. Methods The thesis consists of three papers, based on two separate data collections at Stavanger University Hospital. Paper I and II originate from a retrospective cohort study conducted in 2009, in which women giving birth at Stavanger University hospital in a 4-month period were asked to fill in a questionnaire on demographic features, pain, disability, PGP, pain-related activities of daily living, sick leave in general and for PGP, frequency of exercising before and during pregnancy, and Oswestry Disability Index. Inclusion criteria were singleton pregnancy of at least 36 weeks and competence in the Norwegian language. Drawings of the pelvic and low back area were used for the localization of pain. PGP intensity was then rated retrospectively on a numerical rating scale. Non-parametric tests, multinomial logistic regression and sequential linear regression analysis were used in the statistical analysis. Paper III originate from a prospective longitudinal cohort study carried out in 2010. Inclusion criteria were the as for the retrospective data collection and took place at the second-trimester routine ultrasound examination. All eligible women (n=503) filled in questionnaires and answered a weekly SMS question during pregnancy until delivery. Women with pain in the pelvic area underwent a clinical examination following a test procedure recommended in the European guidelines for the diagnosis and treatment of PGP. Results Paper I report that nearly 50% of the women experienced moderate and severe PGP during pregnancy. Approximately half of them had PGP syndrome, whereas the other half experienced lumbopelvic pain. Ten percent of the women experienced moderate and severe LBP alone. These pain syndromes increased sick leave and impaired general level of function during pregnancy. Approximately 50% of women with PGP had pain in the area of the symphysis pubis. The analysis of risk factors did not present a unidirectional and clear picture. In Paper II PGP is reported to be a frequent and major cause of sick leave during pregnancy among Norwegian women, which is also reflected in activities of daily living as measured with scores on all Oswestry disability index items. In the multivariate analysis of factors related to sick leave and PGP were work satisfaction, problems with lifting and sleeping, and pain intensity risk factors for sick leave. Also, women with longer education, higher work satisfaction and fewer problems with sitting, walking, and standing, were less likely to take sick leave in pregnancy, despite the same pain intensity as women being on sick leave. In Paper III, 42% (212/503) reported pain in the lumbopelvic region and 39% (196/503) fulfilled the criteria for a probable PGP diagnosis. 27% (137/503) reported both the posterior pelvic pain provocation (P4) and the active straight leg raise (ASLR) tests positive at baseline in week 18, revealing 7.55 (95% CI 5.54 to 10.29) times higher mean number of days with bothersome pelvic pain compared with women with both tests negative. They presented the highest scores for workload, depressed mood, pain level, body mass index, Oswestry Disability Index and the number of previous pregnancies. Exercising regularly before and during pregnancy was more common in women with negative tests. Conclusions Pelvic pain in pregnancy is a health care challenge in which moderate and severe pain develops rather early and has important implications for society. The observed associations between possible causative factors and moderate and severe LBP and PGP in the analysis of the retrospective data may, together with results from other studies, bring some valuable insights into their multifactorial influences and provide background information for future studies. Some pregnant women with PGP show a higher pain tolerance, most likely dependant on education, associated with work situation and/or work posture, which decreases sick leave. These issues are recommended to be further examined in a prospective longitudinal study since they may have important implications for sick leave frequency during pregnancy. If both P4 and ASLR tests were positive mid-pregnancy, a persistent bothersome pelvic pain of more than 5 days per week throughout the remainder of pregnancy could be predicted. Increased individual control over work situation and an active lifestyle, including regular exercise before and during pregnancy, may serve as a PGP prophylactic.