This study investigated whether a short course of subcutaneous low-molecular-weight heparin (LMWH) might modify the performance of uterine artery Doppler to predict preeclampsia and small-for-gestational age (SGA) newborns in a high-risk population. A controlled, open-labeled study included 94 women with gestational hypertension and 30 healthy women enrolled at 24 to 26 weeks gestation. Doppler evaluation of uterine arteries resistance index (RI) was performed before and after a two-week course of LMWH (enoxaparin, 4000 IU/d, n = 56 hypertensive patients) or no treatment (n = 38 hypertensive women and 30 healthy controls). There was a significant decrease of uterine artery RI after LMWH (p < 0.001, paired Student's t-test), whereas the untreated hypertensive patients and the healthy control group showed no change between the two Doppler evaluations. The change induced by LMWH was restricted to women with normal outcome, whose RI decreased from (mean +/- standard error) 0.62 +/- 0.01 to 0.56 +/- 0.01 (p < 0.0001). By consequence, the second RI measurement, performed after LMWH administration, had fewer false positive results and higher positive likelihood ratios (LR+) to predict both preeclampsia (LR + 5.91) and SGA (LR + 4.69) compared with the first Doppler examination (LR + 1.97 and 2.22, respectively). Thus, LMWH improved the performance of uterine artery RI to predict preeclampsia and SGA in women with gestational hypertension.