Placental abruption is the premature separation of all or even just a part of the placenta from the uterine wall, resulting in hemorrhage, or bleeding.
This usually happens after about 20 weeks of gestation, and affects about 1% of pregnancies worldwide.
The placenta forms where the embryo attaches to the uterine wall and it’s a unique organ because it develops from both the mom and the fetus, and it’s job is to permit gas and nutrient exchange between them.
The word “placenta” literally means “flat cake.” So picture it as a cake with two layers, the maternal layer and a fetal layer.
The maternal layer, the decidua basalis, is literally a flattened out bag of blood with uterine arteries delivering blood in and uterine veins pulling blood out.
But unlike other parts of the circulatory system where blood stays within narrow blood vessels, the decidua basalis is a huge pool of blood.
The fetal layer of the placenta on the other hand is called the chorion, which is a tissue that has fingerlike projections called chorionic villi which contain tiny fetal arterioles and venules.
These villi push into the decidua basilis, like tiny fingers reaching into a warm pool of oxygen-rich maternal blood.
Gases and nutrients move back and forth between the decidua basalis and the fetal veins, by diffusing through the tissue layer of the thin chorionic villi.
Placental abruption happens when there is a separation of the uterine wall and decidua basalis.
This separation is usually caused by degeneration of the uterine arteries that supply blood to the placenta typically from chronic problems like smoking or hypertension.
These diseased vessels rupture, causing hemorrhage and separation of the placenta.
If the separation is near the margin of the placenta, it can cause vaginal bleeding, but if the separation is more central, there might be a pocket of blood that stays concealed between the decidua basalis and the uterine wall.