We aimed to investigate whether the participation in an observational study on breastfeeding (Doal) modified breastfeeding outcomes in enrolling neonatal intensive care units (NICUs). This bi-centric before-and-after study included neonates who were admitted during a 4-month period before and a 4-month period after the implementation of Doal. Breastfeeding intention and breastfeeding rates at discharge were compared between the two periods. The association between inclusion in Doal and breastfeeding at discharge was assessed among the infants fulfilling the inclusion criteria of Doal. The present study included 655 neonates. After adjustments, both breastfeeding (aOR 1.21, 95%CI [1.1; 1.4], p = 0.001) and exclusive breastfeeding (aOR 1.8, 95%CI [1.4; 2.3], p < 0.001) at discharge increased in the period after. Breastfeeding intention was higher in one center in the period after (79%) compared to before (59%, p = 0.019). Compared to the period before, neonates who were not included in Doal in the period after had a lower chance to be breastfed at discharge, whereas those included were more frequently exclusively breastfed. The participation in an observational study on breastfeeding was associated with an increase in breastfeeding outcomes in enrolling neonatal intensive care units (NICUs). Patients who are not included deserve attention as they are at risk to be disadvantaged regarding breastfeeding success.