Inadequate absorption of intramuscular anti-D immunoglobulin in a pregnant woman with a high body mass index

Introduction: The D antigen is highly immunogenic, and alloanti-D antibodies in maternal blood may cause haemolytic disease of the foetus and newborn. Prophylactic administration of anti-D immunoglobulin (Ig) to D-negative pregnant women significantly reduces the risk of alloimmunisation. However, absorption after intramuscular (IM) injection may be impaired in obese women. Case description: We report the case of a 27-year-old D-negative pregnant woman body mass index (BMI) 35 kg/m2 who received three IM injections of anti-D Ig during pregnancy and after caesarean section. Despite confirmed administration, her indirect antiglobulin tests (IATs) remained negative or only borderline positive. Investigations excluded weak/partial D variants, foetomaternal haemorrhage, chimerism, and other causes. Due to her high BMI, we hypothesised that the injections were administered into subcutaneous fat, resulting in minimal absorption. After intravenous administration of anti-D Ig, the IAT became strongly positive, confirming correct drug delivery. Conclusion: This case highlights the importance of considering maternal BMI in the administration of anti-D Ig. In obese women, inadequate IM administration may compromise prophylaxis. Intravenous administration or the use of alternative injection sites should be considered.