From weeks 5-6 of the pregnancy, foetal trophoblast cells begin to invade the maternal spiral arteries, accumulate and form plugs in the lumen of the vessels. The plugs only allow blood plasma to pass resulting in a physiological oxygen gradient between mother and foetus. These trophoblast plugs block spiral arteries until the beginning of the second trimester (11–14 weeks of pregnancy). In parallel, uterine glands are invaded and opened. This enables the so-called histiotrophic nutrition of the embryo prior to the onset of maternal blood flow into the placenta.
As pregnancy progresses around week 11, the plugs will be dissolved and the blood supply of the placenta with maternal blood starts. Maternal blood will then be present in the intervillous space.
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