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.