Women with pre-gestational diabetes face additional challenges after birth as they struggle with breastfeeding and managing unpredictable blood glucose levels. The aim of this study is to validate the Diabetes and Breastfeeding Management Questionnaire (DBM-Q). In total, 142 mothers with type 1 diabetes mellitus answered the questionnaire, which initially consisted of 11 items. The response rate was 82.5% (n = 128) at two months, and 88.4% (n = 137) at six months postpartum. The measurement properties of the Diabetes and Breastfeeding Management Questionnaire were tested according to the Rasch measurement theory (RMT). One item showed both disordered thresholds and several model misfits and was removed. Two items showed disordered thresholds which were resolved by collapsing response categories. This resulted in a 10-item questionnaire with all the fit residuals within the range of +2.5, minor significant differential item functioning, well-targeted items and a person separation index of 0.73. Evaluating the DBM-Q according to the RMT is a strength, as it evaluates data against strict measurement criteria. This study provides an initial validation of the questionnaire. The DBM-Q shows good measurement properties for measuring diabetes and breastfeeding management postpartum in women with pre-gestational diabetes. Further studies are needed to identify cutoffs for when professional support is needed. © 2020 by the authors.