“Chorio” in pregnancy is short for chorioamnionitis, an infection of the membranes surrounding the baby and the amniotic fluid. It develops when bacteria travel upward from the vagina or cervix into the uterus, triggering inflammation in the chorion and amnion (the two layers of tissue that form the amniotic sac). The hallmark sign is a fever during labor, and the condition requires antibiotics and, typically, delivery of the baby.
What Chorioamnionitis Actually Is
The name comes from the two membranes it affects: the chorion (outer layer) and the amnion (inner layer) that together make up the sac holding the baby and amniotic fluid. When bacteria colonize this space, the body mounts an inflammatory response that can affect both the pregnant person and the baby. Clinically, you may also hear it called “intraamniotic infection.” A panel of experts proposed renaming it “Triple I,” standing for intrauterine infection or inflammation or both, to better capture the range of what’s happening. Most doctors still use “chorio” or “chorioamnionitis” in practice.
There’s also a distinction between clinical and histological chorioamnionitis. Clinical chorioamnionitis is what your care team diagnoses based on symptoms like fever and a fast heart rate. Histological chorioamnionitis is identified after delivery when a pathologist examines the placenta under a microscope and finds signs of inflammation, sometimes without any obvious symptoms during labor. This version can reflect sterile inflammation rather than a true bacterial infection.
How the Infection Develops
Chorioamnionitis is almost always an ascending infection, meaning bacteria move upward from the vagina and cervix into the uterine cavity. The bacteria most commonly involved include Group B Streptococcus, E. coli, Ureaplasma, Gardnerella, and Bacteroides. Often more than one type of bacteria is present.
Several factors raise the risk. Prolonged rupture of membranes (when your water has been broken for an extended time) gives bacteria more opportunity to migrate upward. Longer labors carry more risk for the same reason. Multiple vaginal exams during labor can also introduce bacteria closer to the uterine environment. Being pregnant for the first time and having a labor that requires internal monitoring devices may contribute as well.
Signs and Symptoms
Fever is the most important warning sign. A temperature above 100.4°F (38°C) during labor is considered abnormal and prompts evaluation. The American College of Obstetricians and Gynecologists uses specific thresholds: a single temperature of 102.2°F (39°C) or higher, or a temperature between 100.4°F and 102.1°F that persists when rechecked 30 minutes later.
Beyond fever, other signs that raise suspicion include:
- Maternal heart rate above 100 beats per minute
- Fetal heart rate above 160 beats per minute on the monitor
- Uterine tenderness, where the top of the uterus is painful to touch
- Foul-smelling or cloudy amniotic fluid
Not every symptom needs to be present. An epidural can cause a mild fever on its own, which is one reason care teams look at the full picture rather than fever alone. When fever is the only finding, it’s categorized as “isolated maternal fever” rather than confirmed infection.
How It’s Diagnosed
Most of the time, chorioamnionitis is diagnosed clinically, meaning your doctor makes the call based on your symptoms and vital signs during labor rather than waiting for lab results. Blood work showing elevated white blood cell counts can support the diagnosis, though white cells naturally rise during labor, making this less reliable on its own.
In some situations, an amniocentesis may be performed to sample the amniotic fluid directly. The fluid can be tested with a Gram stain (which shows bacteria under a microscope), cultured for bacterial growth, and checked for glucose levels and other markers. Low glucose in amniotic fluid suggests bacteria are consuming it, pointing toward infection. However, amniocentesis is not routine for this diagnosis and is typically reserved for uncertain cases, especially those arising before labor begins.
A confirmed diagnosis, in the strictest sense, comes after delivery when the placenta is sent to pathology. But treatment decisions are made well before that, based on clinical signs.
Treatment During Labor
Once chorioamnionitis is suspected, antibiotics are started right away, during labor, rather than waiting until after delivery. Early treatment significantly reduces complications for both the mother and the baby. The standard approach involves intravenous antibiotics that cover the most likely bacteria.
Delivery is the definitive treatment. Chorioamnionitis on its own is not an automatic reason for a cesarean section. Vaginal delivery is still the goal when possible, and labor is allowed to progress. However, if labor stalls or other complications arise, a C-section may become necessary. The key principle is that the baby needs to be delivered in a timely manner once infection is identified, because the longer the baby remains in an infected environment, the higher the risk of complications.
Risks to the Baby
Chorioamnionitis raises the odds of several newborn complications by roughly 2 to 3.5 times compared to births without infection. The most immediate concern is early-onset neonatal sepsis, a bloodstream infection in the first days of life. Babies born to mothers with clinical chorioamnionitis have nearly seven times the odds of confirmed early-onset sepsis compared to babies born without it. Other short-term risks include pneumonia and meningitis.
The neurological effects are the most concerning long-term risk. Multiple studies have linked chorioamnionitis to brain injuries in newborns, particularly damage to the brain’s white matter. This type of injury can lead to cerebral palsy, and the connection is especially strong in babies born preterm. Inflammatory molecules released during the infection appear to directly affect developing brain tissue, reducing blood flow and damaging the cells responsible for insulating nerve fibers. Research has also found associations with speech delays and hearing loss at 18 months in very preterm infants exposed to chorioamnionitis.
It’s worth noting that these serious outcomes, while real, are not inevitable. Many babies born after chorioamnionitis do well, particularly when infection is caught early and antibiotics are given promptly. Preterm babies face substantially higher risks than those born at or near full term.
Risks to the Mother
For the birthing parent, chorioamnionitis increases the chance of postpartum complications. Uterine infection after delivery (endometritis) is more common, as are wound infections after cesarean sections. In rare cases, the infection can progress to maternal sepsis, a dangerous whole-body inflammatory response. Excessive bleeding after delivery (postpartum hemorrhage) is also seen more frequently. With prompt antibiotic treatment and delivery, most mothers recover fully.
Prevention and Screening
There is no guaranteed way to prevent chorioamnionitis, but several measures reduce the risk. Group B Streptococcus screening between 35 and 37 weeks of pregnancy is standard. If you test positive, you receive IV antibiotics during labor to prevent the bacteria from reaching the baby, which also lowers the chance of related uterine infections.
During labor, limiting the number of vaginal exams after the membranes have ruptured helps reduce bacterial entry. When membranes rupture before labor starts (your water breaks early), your care team will balance the benefits of waiting for labor to begin naturally against the rising infection risk as time passes. If screening results aren’t available when labor starts and your water has been broken for 18 hours or more, or you develop a fever, antibiotics are typically given as a precaution.