The study hypothesis is SpA may be misdiagnosed on MRI in post partum women due to increased stress on the joint following pregnancy and childbirth, as MRI of the SI joints may show lesions that could mimics axial spondyloarthritis. The primary objective of this study is to determine the incidence of SI lesions seen at MRI in women. The secondary objectives are to compare the incidence and pattern of MRI inflammatory and structural lesions of the SI joints and to determine the factors associated with inflammatory or structural changes of the SI joints in a population of women. the investigators will especially compare the population of women within a year after childbirth versus the population of women who never gave birth or who gave birth over 24 months ago or more.