Context Reports evaluating a possible association between necrotising enterocolitis (NEC) and blood transfusion have been predominantly case–control studies. As the possible associations of disease with any variable on which cases and controls have been matched cannot be explored, a cohort study would offer a solution to this problem.

Objective Our objective was to evaluate the association between exposure to a packed red blood cell (PRBC) transfusion and development of NEC in a cohort where biases of matching are omitted.

Design In a retrospective cohort, exposed infants were defined as those who received a transfusion and did not develop NEC or developed NEC within 48 h of the transfusion. All others were considered unexposed.

Setting A single regional perinatal centre in Memphis, Tennessee, USA.

Patients 3060 ≤1500 g birth weights (BW) were included.

Outcome measures The relative risk of developing NEC after exposure to a PRBC transfusion was measured.

Results 3060 infants were identified. 174 infants (5.7%) developed NEC; 116 of the 174 infants (67%) were exposed. NEC infants had a significantly lower BW (924 vs 1042 g) and required a longer stay on a ventilator (7 vs 2 days). Divided into groups, infants with BW ≤750 , 751–1000 , 1001–1250 g and 1251–1500 g (n=52, 51, 46 and 25, respectively) had a relative risk of 0.14, 0.46, 1.83 and 1.78 (p<0.01, 0.02, 0.07 and 0.17), respectively, to develop NEC after an exposure. Infants with longest ventilator days were also significantly less likely to develop NEC after an exposure; relative risk=0.11 (p<0.01).

Conclusions Exposure to transfusions was less likely associated with NEC in ≤1000 g infants and remained a risk factor in 1001–1500 infants. BW has to be factored in any study evaluating the association between PRBC transfusions and NEC.