A longitudinal study of maternal dose response to low molecular weight heparin in pregnancy.

OBJECTIVE: To assess the maternal response to low molecular weight heparin during pregnancy, by estimation of plasma anti-Xa activity, at three specified gestation points and in the nonpregnant state.
METHODS: A longitudinal, prospective, observational study was set in a tertiary referral recurrent miscarriage clinic. Twenty-four women, attending consecutively, were invited to participate and gave informed consent. Each woman had a history of recurring pregnancy loss and positive preconception screening for antiphospholipid syndrome. After confirmation of a viable pregnancy all subjects began taking 5000 IU of dalteparin once daily subcutaneously. Serial measurement of plasma anti-Xa activity after administration of dalteparin was performed at three standard gestation points (12, 24, and 36 weeks) and in the nonpregnant state (6 weeks postpartum).
RESULTS: Peak anti-Xa levels occurred at 4 hours postbolus in pregnancy, as compared with 2 hours in the nonpregnant state. The mean anti-Xa levels at 12, 24, and 36 weeks' gestation were significantly reduced, at 2 hours postinjection, as compared with the nonpregnant state (P <.001, P <.01, P <.001, respectively). The lowest dose-response curve was at 36 weeks' gestation. A repeated-measures analysis of variance found a significant difference (P <.05) between the 36-week group and the postterm group but not between any of the other groups.
CONCLUSION: During pregnancy, differences in the pharmacokinetics of low molecular weight heparin were observed, with an overall reduction in anti-Xa activity. On the basis of this study it is questionable to extrapolate dosing and lack of dose monitoring, in pregnant women, using data derived from a nonpregnant population.