Background: Low back and pelvic pain (LBPP) is an important issue related to mental health as well as to difficulty for daily life activity during early pregnancy and in the postpartum period. We examined the intensity of LBPP, difficulty for daily life activity and mental health and their associations from early pregnancy until 3 months postpartum in a prospective study. We also examined predictive factors for LBPP at 3 months postpartum. Methods: We recruited 55 pregnant women who responded to questionnaires in the first, second and third trimesters and at one week, one month and 3 months postpartum. We designed a self-administered questionnaire including a visual analog scale (VAS), pregnancy mobility index (PMI) of LBPP and Edinburgh postnatal depression scale (EPDS). Results: There were significant differences in the proportions of women with LBPP in the first trimester (63.6%), second trimester (78.2%) and third trimester (83.6%) and at one week postpartum (80.0%), one month postpartum (69.1%) and 3 months postpartum (56.4%). VAS score at 3 months postpartum was significantly correlated with VAS scores at the first trimester (r = 0.410, p = 0.002), second trimester (r = 0.298, p = 0.027) and one month postpartum (r = 0.476, p < 0.001). VAS score at 3 months postpartum was significantly correlated with PMI scores at the first trimester and one month postpartum.