ObjectiveTo determine the changes in the vessel-wall thickness and the radius of the lumen in tertiary-stem villi of the placenta with advancing gestational duration and their relationship to umbilical artery Doppler studies.MethodsPlacentas from 63 miscarriages and preterm and term deliveries (between 19 and 40 weeks) were used for morphometric study of the tertiary-stem villi vessels. Each women had undergone Doppler flow study of the umbilical arter. The resistance index (RI) was determined from the Doppler flow velocity waveform. Placental paraffin sections of 4-μ thickness were stained with hematoxylin and eosin and with periodic acid-Schiff reagents. The tertiary-stem villi and their vessels were examined microscopically and assessed morphometrically using a personal computer with math co-processor and a touch-sensitive screen overylying a video monitor. The monitor received microscopic images from a video camera that was mounted on a microscope. We determined vessel-wall thickness by tracing the outer and inner circumferences of digitized vessel-wall images.ResultsWall thickness, but no lumen size, of the tertiarystem villi vessels decreased significantly overall at a rate of 0.63μ/week (P< .001). The rate of decrease was 0.64μ/week (P< .001) during the second trimester and 0.50μ/week (P< .001) during the third trimester. There was a significant correlation between the decrease in thickness and in RI (r= .083) [P< .001],r= 0.78 [P< .001] in the second and third trimesters, respectively). Resistance indices were all within normal limits.ConclusionPlacental tertiary-stem villi vessel-wall thickness decreases with advancing gestational age. There is a correlation between the changes in RI of the umbilical artery Doppler flow and the changes in mean wall thickness of the placental vessels.