Chorioamnionitis

Chorioamnionitis is, by definition, inflammation of the sac surrounding the baby in the uterus.  The membrane is more commonly known as the “bag of water.”  It is caused by an infection from bacteria that ascends, or moves up, from the vagina to the uterus after the bag of water has broken. It is typically discovered when a pathologist (a doctor specially trained to examine tissue specimens from the body) looks at samples of the placenta under a microscope after birth. Traditionally, chorioamnionitis is strongly associated with prematurity and prolonged labor. The risk of developing chorioamnionitis is increased when vaginal examinations are performed between 35 weeks of pregnancy to delivery. This includes examinations performed during the labor process, especially after the bag of water, or membranes, have broken.

Signs of chorioamnionitis always include the presence of white blood cells (the type of cells that help fight infection), in the three membranes of the placenta. These three layers are the decidua, chorion, and amnion.

The decidua is part of the lining of the uterus, the chorion is the outer layer of the placenta closest to the mother, and the amnion is the inner layer of the placenta closest to the baby. In a placenta, white blood cells can be found in any of these three membranes. The severity of chorioamnionitis can be determined by the number of white blood cells, or neutrophils, present and where they are located in the placenta.

Histologic chorioamnionitis is defined by the presence of acute neutrophils in the fetal surface of the placenta (the amnion and/or chorion of the membranes). The location of the neutrophils can be defined as amniocentric, meaning that the neutrophils have migrated from the maternal blood toward the amnion cavity, but haven’t necessarily reached the amnion.

Acute chorioamnionitis can be graded as mild, moderate or severe. Mild chorioamnionitis is characterized by the presence of neutrophils in subchorionic space between the decidua and chorion. Moderate chorioamnionitis is characterized by the presence of neutrophils in the connective tissue between the chorion and the amnion. Severe chorioamnionitis is characterized by death of the placental cells, and sloughing, or shedding of the tissues of the amnion, the layer closest to the baby. Quite often, abscesses can form in the layers as well.

Histologic chorioamnionitis may be present in women with or without clinical chorioamnionitis symptoms. Clinical chorioamnionitis, also referred to as clinical intraamniotic infection, occurs when clinical signs are present, including maternal fever, uterine tenderness, maternal tachycardia (fast heartbeat, >100/min), fetal tachycardia (>160/min) and purulent or foul amniotic fluid. It is usually the result of an ascending bacterial infection by group B streptococci, but can also be caused by other bacterium.

Inflammation in the placenta can contribute to hypoxic-ischemic encephalopathy (a condition that occurs when the brain is deprived of oxygen) by decreasing the ability of the placenta to supply oxygenated blood to the baby and remove metabolic waste products from the baby back to the mother.

A related infection that may occur is funisitis, which is an inflammation of the umbilical cord. As the umbilical cord is literally the lifeline from the mother to the baby, funisitis can also contribute to hypoxia-ischemia by causing the cord to spasm or constrict, decreasing oxygen and blood flow between the mother and baby.