Aim Studies indicate that reduced foetal haemoglobin levels are related to increased neonatal morbidity rates. This study investigated the relationships between sampling‐related blood loss and adult blood transfusions administered during postnatal days 1‐14 and the development of severe neonatal morbidities in extremely preterm infants born before 28 weeks of gestation. Methods The medical files of 149 extremely preterm infants born at two university hospitals in Sweden from 2013 to 2018 were investigated. Results Blood sampling resulted in a 58% depletion of the endogenous blood volume postnatal days 1‐14 (median 40.4 mL/kg, interquartile range 23.9‐53.3 mL/kg) and correlated with the adult erythrocyte transfusion volume (rS = 0.870, P < .001). Sampling‐related blood loss on postnatal days 1‐7, adjusted for gestational age at birth and birth weight standard deviation score, was associated with the development of bronchopulmonary dysplasia (BPD) (odds ratio by a 10‐unit increase 2.4, 95% confidence interval 1.1‐5.4) (P = .03). No associations were found between blood sampling and intraventricular haemorrhage or necrotising enterocolitis in the full statistical model. The largest proportion of sampling‐related blood was used for blood gas analyses (48.7%). Conclusion Diagnostic blood sampling led to major endogenous blood loss replaced with adult blood components and was associated with the development of BPD.