Between Weeks 0 and 13, the fertilized blastocyst embeds in the uterine wall, and development of the fetus and placenta begin. At this time, the placenta is a relatively low-oxygen environment. Placental villi are lined with cells known as cytotrophoblasts and syncytiotrophoblasts. The cytotrophoblasts breach the uterine wall and begin reshaping blood vessels there. These remodeled vessels become a source of maternal blood for the placenta. The placenta develops to bring oxygen and nutrients to the fetus and to move harmful waste and nutrients away. Over time, the villi develop increasingly dense branching to accommodate the increased demand of the developing fetus.
Reshaping of spiral arteries
Spiral arteries in the uterine wall are remodeled to supply blood to the fetus during pregnancy. When there is no pregnancy, the spiral artery spans the tissue of the uterine wall, supplying blood to the lining. In a normal pregnancy, cytotrophoblast cells from placental villi invade the space around the spiral artery, replacing cells which normally line the vessel.