The number of uteroplacental vessels flowing into the intervillous space in mature placentas has been reported in some studies to increase and in others to decrease. Both villous infiltration and proliferation in the vessels have been neglected in histologic examinations and need to be studied.
To clarify these conflicting findings regarding villous infiltration and proliferation in the uteroplacental vessels.
A retrospective study was done using placental sections of different gestational ages. The openings of the vessels in each section were counted, noting if they were with or without villous infiltration. The frequency of openings per section was calculated, and the findings were grouped by gestational age.
The frequency of total openings was significantly lower in the 41 to 37 weeks group (1.2) than in the 36 to 30 weeks group (2.0), the 29 to 20 weeks group (2.2), and the 19 to 13 weeks group (2.4) (P < .001). The frequency of openings with villous infiltration decreased similarly in mature placentas. Chorionic villi infiltrated the openings and proliferated in uteroplacental vessels. Differentiation between arteries and veins was difficult in the basal plate, because all vessels had veinlike structures.
This decreasing frequency of openings in mature placentas suggests that the amount of intervillous blood flow is limited in full-term and postterm pregnancy. Villous infiltration into the vessels is considered a normal finding with gestation and can provide trophoblast for uteroplacental arteries. The relationship between placental development and villous proliferation in the vessels is presented.