Introduction Obesity is a risk factor for thromboembolism in pregnancy. Recent RCOG guidelines suggested a non-evidence based weight-dependent protocol in which monitoring of anti-Xa levels was not required. This contrasted with local guidelines which used BMI based dosing, using anti-Xa levels to determine correct dose with BMI > 35. We sought to investigate the impact of the different strategies in a cohort of women treated with antenatal thromboprophylaxis.

Methods We retrospectively audited the thromboprophylaxis practise amongst 42 women between September 2009 and September 2011. We observed tinzaparin dosing, frequency of anti Xa levels, dose changes, and pregnancy outcomes.

Results 39/42 (93%) had a BMI over 35 and had anti-Xa measurements. Using the local protocol 15/39 (38%) required dose increases and all patients received a higher dose than suggested by RCOG guidelines (median 3000 IU, interquartile range(1QR) 3000–5000 IU). There were no thrombotic events and 25/38 (66%) achieved a vaginal delivery. The median estimated blood loss at delivery was 350 ml (IQR 200–725 mls) and 3 women suffered a major PPH > 1500 mls.

Discussion All our patients received a higher tinzaparin dose than suggested by RCOG guidelines, but shown by anti-Xa monitoring to be therapeutic. In these small numbers, there was a high vaginal delivery rate (66%) and although 3 women suffered a major PPH, the median blood loss was within an acceptable range for this high risk population. Without appropriate monitoring, RCOG guidelines may be resulting in suboptimal anticoagulation in women with raised BMI, whilst exposing them to the risks and side effects of LMWH.