OBJECTIVE To investigate the relationship between the timing of disappearance of high-resistance uterine artery waveforms between the first and second trimester of pregnancy and birth weight. METHODS Uterine artery Doppler recordings were obtained in a cohort of singleton pregnancies at 11-14 weeks of gestation and subsequently at 18-23 weeks. At each examination, the presence or absence of an early diastolic notch was recorded for each side. An abnormal pattern of blood flow was defined as the presence of bilateral notches. Cases complicated by preeclampsia or preterm delivery were excluded. RESULTS Four hundred eleven pregnancies showed absent or unilateral uterine artery notches at the 11-14-week scan (group 1). All these cases maintained a low-resistance uterine blood flow pattern at the second-trimester scan. Of the 251 pregnancies with bilateral notches at the 11-14-week scan, 222 subsequently displayed a low-resistance blood flow at 18-23 weeks (group 2) whereas only 29 maintained bilateral notches (group 3). The mean birth weight was higher in group 1 (3452 g) than in groups 2 (3310 g) and 3 (3224 g). This difference was confirmed after adjusting for confounding variables. Small-for-gestational-age fetuses were more common in groups 2 (14.9%) and 3 (14%) compared with group 1 (6.8%, P <.001). During the study period, severe preterm preeclampsia was observed in 6 cases only. CONCLUSIONS The longitudinal variation in uterine artery blood flow pattern has a statistically significant correlation with birth weight, likely reflecting the timing and degree of trophoblastic invasion of the maternal vessels. METHODS II-2