What causes blood clots to form behind the placenta?
When the placenta first implants on the inner uterine wall, it secretes enzymes which dissolve the ends of the capillaries which come to the inner surface of the uterus. As a result, the open ends of the arterial capillaries spout little fountains of blood behind the placenta, and the open ends of the venous capillaries return the blood to the mother’s heart, like little bathtub drains. This is called an arterial-venous shunt. A lake of blood forms behind the placenta, and the baby’s capillaries in the placenta, which remain intact (like little loops), are continually bathed in this lake of the mother’s blood. Through this process, oxygen and nutrients pass from the mother’s lake of blood, through the baby’s capillary walls, and into the baby’s blood stream, and waste products pass from the baby’s capillaries to the mother’s blood. As the placenta grows, the lake of blood behind the placenta needs to grow, and the mother needs to grow more blood in order to keep this lake of blood well
