Pregnant rats were exposed to hypoxia (equivalent to 11% oxygen) for defined intervals during gestation (day 6.5 to day 9.5, panel B; day 10.5 to day 13.5, panel D; day 6.5 to day 13.5, panels F) and then returned to normoxia until day 13.5 of gestation when all animals were sacrificed. Gestationally matched pair-fed rats exposed to ambient conditions were used as controls (panels A, C, and E). Placentation sites were sectioned and stained for ACTA2, identifying smooth muscle-associated blood vessels in the uterine mesometrial compartment. Chromogen: AEC; counterstain: hematoxylin; scale bars = 0.5 mm. Panel G, quantification of the ratio of blood vessel cross-sectional area to uterine mesometrial compartment cross-sectional area (BV:UMC) in pair-fed normoxic controls and hypoxia exposed placentation sites. Values are means ± the standard error of each mean (n=5 per group). The asterisk indicates a significant increase in BV:UMC cross-sectional area following hypoxia exposure (day 6.5 to day 13.5, P<0.005). Abbreviations: PF-N, pair-fed normoxia; HYP, maternal hypoxia.