A 34-year-old woman is reviewed 18 hours after an emergency caesarean section complicated by a 1.6 L postpartum haemorrhage. The bleeding has been controlled for 12 hours, her haemoglobin is 82 g/L (115–155), platelet count is 155 × 10⁹/L (150–400), and she has no neuraxial catheter or renal impairment. She has a body mass index of 38 kg/m² and a previous unprovoked deep-vein thrombosis. Which one of the following is the most appropriate next step?