Weight-adjusted LMWH prophylaxis provides more effective thrombin inhibition in morbidly obese pregnant women

Low molecular weight heparin (LMWH) prophylaxis has been recommended for morbidly obese pregnant women (>40kg/m(2)). There is very little data on the anticoagulant effects of LMWH in this group. We investigated two different dosing regimens; fixed dose and weight-adjusted dose on the anticoagulant effects of the LMWH tinzaparin used for thromboprophylaxis in obese pregnant women.

Twenty morbidly obese pregnant women were started on a fixed dose of tinzaparin (4,500 iu/day) at 32weeks gestation and then changed to a weight-adjusted dose (75iu/kg/day) for the remainder of their pregnancy. Four-hour post LMWH, venous bloods were taken after each initial dose and repeated every two weeks until delivery. Twenty normal weight women who did not receive LMWH at the same gestation were used as controls.

Prior to LMWH prophylaxis, tissue factor pathway inhibitor (TFPI) levels in the obese group at 32weeks were significantly lower (p<0.001) and endogenous thrombin potential (ETP) and peak thrombin levels in obese group were significantly higher, compared with controls (p<0.0001; p<0.001). There was no significant difference between ETP levels before and after fixed LMWH. However, ETP levels were significantly lower post weight-adjusted dose 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 prior to LMWH (r=0.631) (p<0.05) and at fixed dose (r=0.578) (p<0.05).

Morbidly obese pregnant women have increased thrombin generation and reduced natural anticoagulant in third trimester. This prothrombotic state was more effectively attenuated by weight-adjusted than fixed LMWH doses.