Maternal anti‐factor Xa activity following subcutaneous unfractionated heparin after Caesarean section*

Unfractionated heparin is widely used for prophylaxis against venous thromboembolism after Caesarean section. We performed a survey of thromboprophylactic methods after elective Caesarean section in 50 maternity units in the United Kingdom. We found that a variety of regimens were used. Thirteen (26%) used subcutaneous unfractionated heparin at standard (non‐pregnant) doses. We then studied anti‐Xa activity in women following elective Caesarean section under regional anaesthesia. Initially, eight women were given 5000 U unfractionated heparin subcutaneously after surgery and anti‐Xa activity was measured 1, 2, 3, 4, 5, 6, 8 and 10 h after administration. There was no detectable anti‐Xa activity in any of the samples so the dose was increased to 7500 U in a further five women and a single anti‐Xa assay performed at 3 h when peak activity should occur. Again, no activity was detected so the dose was increased to 10 000 U heparin in a final group of 10 women and anti‐Xa activity measured at 0.5, 1, 1.5, 2, 3, 4, 5 and 6 h. Although there was some activity after 10 000 U heparin, the level was below that accepted for prophylaxis. If anti‐Xa activity is an appropriate monitor of prophylactic unfractionated heparin, doses up to 10 000 U are inadequate. Since there is evidence that enoxaparin is effective at producing adequate prophylactic anti‐Xa activity following Caesarean section, we suggest abandoning the use of unfractionated heparin in favour of enoxaparin for this purpose.