We wanted to determine whether low-dose aspirin has an effect on birthweight in a low-risk population and to evaluate the interaction between aspirin and maternal risk factors in relationship to their effect on birthweight. We also wanted to determine the specific fetal ultrasound measurements affected by low-dose aspirin and the gestational age at which this effect becomes apparent. The women studied were derived from a population of 606 nulliparous women who participated in a randomized trial of low dose aspirin to reduce the incidence of preeclampsia. This analysis included only women who remained normotensive and delivered live horn singletons at term. There were 254 women in the group randomized to 60 mg of aspirin daily from 24 weeks onward, and 248 who received a daily placebo. The mean birthweight was 3,353 g in the aspirin versus 3,282 g in the control group, a difference of 71 g (P = 0.08). In a regression analysis controlling for race, height, weight, smoking, and infant sex, the use of aspirin was associated with an 88-g increase in birthweight (P = 0.04). The effect of aspirin on birthweight in black and white infants and male and female infants was not significantly different. However, when the study population was divided at the median by height and weight, virtually the entire increase in birthweight associated with low-dose aspirin was found in the short and thin women, with birthweight increases in these groups of approximately 140 g (P ≤ 0.02). Sequential fetal ultrasound measurements in thin women revealed significant changes in the abdominal circumference (P < 0.001) at 27 weeks and above associated with aspirin use, but no differences in femur length or head circumference. Low-dose aspirin has a significant impact on birthweight in a low-risk nulliparous population, which is most marked in thin and/or short women. The effect is related predominantly to an increase in the fetal abdominal circumference. These results are compatible with current knowledge about the mechanism of action of low dose aspirin, and the etiology of decreased birthweight in thin women. © 1995 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.