Background
O Rh(D)− red blood cell (RBC) units can generally be transfused to most patients regardless of their ABO blood type and are frequently used during emergency situations. Detailed usage patterns of O Rh(D)− RBC units in obstetric populations have not been well characterised. With the introduction of patient blood management guidelines, historical usage patterns are important for providing comparative data.

Aims
To determine how the use of O Rh(D)− RBC units in pregnant women differs between hospitals of different sizes and obstetric capabilities prior to patient blood management guidelines.

Methods
Data from 67 New South Wales public hospital blood banks were linked with hospital and perinatal databases to identify RBC transfusions during pregnancy, birth and postnatally between July 2006 and December 2010. RBC transfusions were divided into O Rh(D)− or other blood types. Hospitals were classified according to birth volume, obstetric capability and location, with transfusions classified by timing and diagnosis.

Results
Of the 12 078 RBC units transfused into pregnant women, 1062 (8.8%) were O Rh(D)−. Higher use of O Rh(D)− RBC units was seen in antenatal transfusions, preterm deliveries and in regional or smaller hospitals. There was wide variation in rates of O Rh(D)− RBC transfusion among hospitals.

Conclusions
The rate of O Rh(D)− RBC unit use in obstetrics was lower during the period assessed than the nationally reported usage. It is encouraging that O Rh(D)− RBCs were more commonly used in emergency or specialised situations, or in facilities where holding a large blood inventory is not feasible.