The purpose of this study was to establish the time course and hemodynamic significance of de novo formed and enlarged uteroplacental arteries during pregnancy. Using x-ray microcomputed tomography (n = 4–7 placentas from 2–4 dams/gestational group), uteroplacental arterial vascular dimensions were measured at individual implantation sites. Dimensions and topology were used to compute total and vessel-specific resistances and cross-sectional areas. Diameter enlargement of the uterine artery ( 55% by Embryonic Day 5.5 [E5.5]) and preplacental radial arteries ( 30% by E8.5) was significant only in early gestation. Formation of spiral arteries (E9.5–E11.5), maternal canals, and canal branches (E11.5–E13.5) during midgestation was followed by enlargement of these vessels such that, from E9.5 to E17.5 (near term), spiral artery resistance dropped 9-fold, and canal resistance became negligible. A 12-fold increase in terminal vessel cross-sectional area was nearly sufficient to offset known increases in flow so that blood velocity entering the exchange region was predicted to increase by only 2-fold. The calculated 47% decrease in total resistance downstream of the uterine artery, determined from vascular geometry, was in accord with prior uterine blood flow data in vivo and was due to enlarging spiral artery diameters. Interestingly, radial artery resistance was unchanged after E9.5 so that radial arteries accounted for 91% of resistance and pressure drop in the uteroplacental arterial network by E17.5. These findings led us to propose functional roles for the three morphologically defined vessel types: radial arteries to reduce pressure, spiral artery enlargement to increase flow with gestation, and maternal canal elaboration and enlargement to maintain low exit velocities into the exchange region.