Background LMWH prophylaxis has been recommended for morbidly obese pregnant women (>40 kg/m²). However, no data exists on the anticoagulant effects of LMWH in this group.

Aim We investigated different dosing regimens; fixed dose versus weight adjusted dose on the anticoagulant effects of the LMWH, tinzaparin used for thromboprophylaxis in obese pregnant women.

Method Twenty morbidly obese pregnant women were started on a fixed dose of tinzaparin (4,500 iu/day)at 30 weeks gestation and then changed to a weight adjusted dose (75 iu/kg/day) for the remainder of their pregnancy. Four hour post dose venous blood were taken after each initial dose and repeated every 2 weeks until delivery. Twenty normal weight women at the same gestation were used as controls.

Result Prior to LMWH prophylaxis, TFPI levels in the obese group at 30 weeks were significantly lower (p < 0.001) and ETP and peak thrombin levels in obese group were significantly higher compared with controls (P < 0.0001; P < 0.001).

Within the obese group, there was no significant difference between ETP levels before and after fixed LMWH dose. However, ETP levels were significantly lower post weight-adjusted dose (75 iu/kg tinzaparin) compared with post fixed dose. There was a significant effect of LMWH on TFPI levels, (p < 0.0001). ETP correlated positively with total body weight at fixed dose (r = 0.578) (p < 0.05).

Conclusion Morbidly obese pregnant women have increased thrombin generation and reduced natural anticoagulant in third trimester. The prothrombotic state in pregnant morbidly obese women was substantially attenuated by weight adjusted but not at fixed LMWH doses.