When that baby no longer occupies the space inside the uterus, the uterus contracts down. Like a balloon that's been untied, the amount of surface area within gets smaller. Since the placenta is attached to the uterine wall, a property dependent on the original implantation of the blastocyst, this attached area gets smaller, causing the uterus to crimp. The separation of the placenta results primarily from the disproportion created between the unchanged size of the placenta and the reduced size of the underlying implantation site. And with the end of the third stage of labor, we say goodbye to the most important organ we've ever had.
Calcification is a sign of placental aging.
In fact the amount of calcification, seen as white specks on the ultrasound, can tip off a doctor as to how near term a pregnancy is. But it can also be a sign of premature aging, which will decrease the amount of nutrition and oxygen going to the baby. The more calcification, the less placenta is normal tissue for supplying the baby. Don't panic, though. The placenta has a lot of slack.