Amnionitis is an infection of the uterus, the amniotic sac (bag of waters), and in some cases, of the fetus. Amnionitis is very rare, occurring in only about 5% of pregnancies. The uterus is normally a sterile environment (meaning that it does not contain any bacteria or viruses). However, certain conditions can make the uterus susceptible to infection. When it does occur, infection of the uterus is a serious condition because it cannot be successfully treated without delivering the baby. This is a particular problem when the baby is premature.

Infection of the uterus is caused by bacteria invading the uterine cavity from one of two routes. Rarely, bacteria will enter the uterus through the mother's bloodstream. The more common route is from the vagina and cervix. In healthy women, the vagina and cervix always contain limited numbers of bacteria. In certain people, however, these bacteria can cause infection.

Risks for amnionitis include preterm labor, rupture of membranes, and a dilated cervix. These can allow bacteria in the vagina to gain access to the uterus. Preterm premature rupture of membranes (PPROM-water breaking before 37 weeks) presents the highest risk for amniotic infection.

Symptoms of amnionitis are variable. One of the earliest signs can be regular contractions with cervical dilation; these symptoms together signify the start of preterm labor. A flu-like feeling may also be noted at this early stage. As the infection progresses, the abdomen (uterus) can become tender to touch and fever may develop. Laboratory tests may show a rise in the white blood cell count. If the infection is not treated, the fetus may become sick and the fetal heart rate may increase (this is not obvious unless the mother is in a hospital and connected to a fetal heart rate monitor). Without treatment, the mother may go into preterm labor. In rare cases, a serious infection could lead to fetal death. The mother can also become very sick and may develop sepsis. Sepsis is when the infection enters the mother's bloodstream causing problems in other parts of the body. Sepsis also causes serious blood pressure problems due to overwhelming numbers of bacteria-releasing toxins into the bloodstream and causing the blood vessels to relax.

Amnionitis can also occur during normal labor. Factors that can increase the risk for amnionitis include:

  • a long labor;
  • prolonged rupture of membranes;
  • multiple vaginal exams;
  • placement of fetal scalp electrodes; and
  • intrauterine pressure catheters.

The diagnosis of amnionitis in labor is based on the presence of fever, uterine tenderness, increased white blood cell count, and foul-smelling amniotic fluid. Amniocentesis is not used to diagnose amnionitis during normal labor. Antibiotics should be given as soon as possible after the diagnosis is made to lower the risk for both the mother and fetus. When infection is diagnosed during labor, efforts should be made to shorten labor as much as possible; this is usually achieved with the use of oxytocin (Pitocin) to strengthen the contractions. Amnionitis can also be a cause of dysfunctional labor, despite the use of oxytocin.