Background: The risk of venous thromboembolism (VTE) and arterial thrombosis (stroke and heart attack) increases in women during pregnancy. Enoxaparin (a low molecular weight heparin) and unfractionated heparin are extensively used in the VTE prophylaxis and therapeutic doses.Aims: We retrospectively describe an apparent relationship between enoxaparin and unfractionated heparin (UFH) in a woman with gestational thrombocytopenia developing a hypersensitivity reactions.Case presentation: we report a clinical scenario of a 43 years old pregnant women (gravida 18, para 12 and living 6) with co-morbid obesity (BMI 47.9) and well known to have a gestational thrombocytopenia during the last two pregnancies. She was administered enoxaparin 0.4ml (40 IU) daily for VTE prophylaxis. After three days, the patient came with hypersensitivity reactions like erythema, pain, swelling, irritation at the site of injection and rash all over the body (lower limbs, abdomen and neck) appeared after the first dose of enoxaparin. Hypersensitivity skin reaction to enoxaparin was suspected and immediately drug was withdrawn. The performed UFH skin test was negative, and then she was prescribed UFH 5000 units subcutaneous every 12 hours. The patient had similar reaction but a severe form and UFH was discontinued. Fondaparinux was the only nonheparin anticoagulant available and was prescribed for this patient.Conclusions: The patient had an allergic drug reaction and provides an evidence of cross-allergenicity between enoxaparin and UFH. Nonheparin anticoagulant improves both rashes and gestational thrombocytopenia.