Figure 1.

Potential route of maternal blood flow into the intervillous space during first trimester. During early placental development, maternal blood flow in maternal uterine spiral arteries is obstructed by extravillous trophoblast plugs. However, parts of maternal blood (red arrows) can pass through narrow intertrophoblastic gaps when trophoblast plugs begin to dissolve during first trimester of gestation. The laminar blood flow from the maternal spiral artery changes to a turbulent flow upon entrance into the intervillous space.