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To estimate the angiogenic effect of heparin on human umbilical vein endothelial cells cultured in conditioned media from normal and severely pre-eclamptic human placental villi.Normal first- and second-trimester floating placental villi were explanted in control conditions and increasing concentrations of heparin (unfractionated and low molecular weight heparin) across the clinical prophylactic and therapeutic range (0.025–25 units/mL). At 96 hours, the placenta-conditioned media was tested for angiogenic activity in a human umbilical vein endothelial cell in vitro angiogenesis assay. Total capillary-like tube length and number of branch points were determined from photographs that did not contain information about experimental conditions. The response of placenta-conditioned media from preterm severely preeclamptic pregnant women exposed to low molecular weight heparin also was assessed and compared with both preterm and term control groups.Unfractionated heparin significantly promoted angiogenesis (0.25 units/mL compared with control: relative branch points 185±32% [mean±standard error of the mean], P<.05), whereas low molecular weight heparin had no significant effect. Addition of unfractionated or low molecular weight heparin to first- and second-trimester placenta-conditioned media significantly promoted angiogenesis with the response to low molecular weight heparin more than double that of unfractionated heparin (low molecular weight compared with unfractionated heparin at 2.5 units/mL: relative branch points 930±158% compared with 398±90%, P<.05). Placenta-conditioned media from pregnancies with severe preeclampsia arrested angiogenesis in comparison with both preterm and term pregnancies and was not significantly restored by the addition of low molecular weight heparin.Unfractionated and low molecular weight heparin promote in vitro angiogenesis in healthy first- and second-trimester placenta-conditioned media. The nonanticoagulant actions of heparin may be relevant to the prevention of severe preeclampsia.