The significance of ambulatory blood pressure (ABP) monitoring during pregnancy has not been established. We performed a prospective study to elucidate whether ABP measures are associated with small-for-gestational-age birth weight (SGA). We studied 146 pregnant women who were seen for maternal medical checkups or suspected hypertension. ABP monitoring was performed for further assessment of hypertension. The outcome measure was SGA. The subjects were classified by their medical history and ABP as having preeclampsia or gestational hypertension (n=68 cases), chronic hypertension (n=48) or white-coat hypertension (n=30). There were 50 (34.2%) cases of SGA by the fetal growth reference standard. In multivariable logistic regression analyses adjusting for age, body mass index, the presence of prior pregnancy, current smoking habit and the use of antihypertensive medications, 24-h SBP (per 10 mm Hg (odds ratio (OR): 1.74; 95% confidence interval (CI): 1.28-2.38; P<0.001)) was more closely associated with SGA than clinic BP (OR: 1.40; 95% CI: 0.92-2.13; P=0.11). The results were essentially the same if 24-h BP was replaced by awake or sleep SBP. Ambulatory diastolic BP showed the same tendency. However, abnormal circadian rhythm was not associated with the outcome. In conclusion, ambulatory BP monitoring measures performed during pregnancy were more closely associated with SGA than clinic BP.