A prospective randomized, double-blinded, controlled clinical trial was conducted in 84 pregnant women with high risk of intrauterine growth retardation (IUGR). From the 28th to 30th week of gestation, low dose aspirin 75 mg/day (study group, n = 40) or placebo (control group, n = 44) was given consecutively for 6 to 8 weeks. It was found that the mean value of systolic/diastolic ratio of umbilical artery flow velocity waveforms in the study group was significantly lower than that of the control group after drug use. The occurrences of IUGR and preeclampsia in the study group (7.5% and 10.0% respectively) were remarkably lower than those in the control group (27.3% of both). The adverse effects of low dose aspirin on both mother and fetus were not observed. These results suggest that low dose aspirin administration (75 mg/day) beginning at earlier stage of third trimester may improve the fetal placental circulation, and thus prevent IUGR and/or preeclampsia effectively.