Increased unfractionated heparin requirements with decreasing body mass index in pregnancy

Pregnant women receiving low-molecular-weight heparin for therapeutic anticoagulation are often converted to unfractionated heparin in anticipation of labor. We aim to characterize the impact of maternal body mass index on attainment of target anticoagulation during the conversion process.

We conducted a five-year retrospective study of a pregnancy cohort converted from low-molecular-weight heparin to unfractionated heparin in the third trimester. Patient demographics, anticoagulation regimens, and clinical outcomes were extracted from the medical record. Nonparametric statistical methods were used for analysis by body mass index (<30, 30-35, and >35).

Thirty-one subjects were evenly distributed by body mass index (p = 0.97). Linear regression revealed an inverse correlation between patient body mass index and unfractionated heparin dose needed to achieve therapeutic anticoagulation (p = 0.04). Subjects with body mass index > 35 attained therapeutic activated partial thromboplastin time levels at 18 U (Units)/kg/h, while subjects with body mass index < 30 required 25 U/kg/h (p = 0.02).

Higher doses of unfractionated heparin are needed to achieve anticoagulation in patients with body mass index < 30 during pregnancy. This paradoxical relationship may be explained by physiologic characteristics that increase unfractionated heparin elimination, including diminished adiposity and increased renal clearance.