Objective: Postpartum social problems, such as postpartum depression and bonding disorders, are important risk factors for child maltreatment. Mothers with such problems are known to need social support. The aim of this study was to develop and validate the Social Life Impact for Mother (SLIM) scale to identify mothers in Japan who need social support during postpartum. Design: A hospital-based prospective study. Setting: Obstetric clinics and hospitals in four populous prefectures in Japan. Sample: A total of 7462 pregnant women. Methods: The participants completed the SLIM scale at first trimester, and postpartum social problems (postpartum depression and bonding disorders) were assessed at one month after delivery (N=5768, follow-up rate: 77.3%). Multivariate logistic regression was applied to investigate the association between the SLIM scale and postpartum social problems. Main outcome measures: Postpartum social problems (postpartum depression and bonding disorders) at one month after delivery. Results: The SLIM scale predicted postpartum social problems in moderate accuracy (AUC=0.63, 95% confidence interval: 0.60-0.65). Further stratification by local clinic and tertiary hospital did not affect the estimates. Conclusion: The SLIM scale at prenatal checkup may be useful for obstetricians to detect mothers with postpartum social problems. Further intervention study using SLIM score is warranted.