Nonneoplastic placental conditions and abnormalities


Acute chorioamnionitis

Editorial Board Member: Ricardo R. Lastra, M.D.
Deputy Editor-in-Chief: Jennifer A. Bennett, M.D.
Andrew P. Norgan, M.D., Ph.D.
Drucilla J. Roberts, M.D.

Last author update: 16 June 2022
Last staff update: 16 June 2022

Copyright: 2002-2023,, Inc.

PubMed Search: Acute chorioamnionitis

Andrew P. Norgan, M.D., Ph.D.
Drucilla J. Roberts, M.D.
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Cite this page: Norgan AP, Roberts DJ. Acute chorioamnionitis. website. Accessed June 4th, 2023.
Definition / general
  • Acute chorioamnionitis is defined by the presence of acute inflammation (neutrophils) within the chorion or amnion (or both) of the extraplacental membranes or chorionic plate (maternal inflammatory response [MIR]), with or without acute inflammatory cell extravasation from the umbilical cord vasculature or chorionic plate vessels (fetal inflammatory response [FIR])
Essential features
  • Acute chorioamnionitis is due to a maternal (with or without accompanying fetal) inflammatory response
  • Fetal inflammatory response (inflammatory cell extravasation from umbilical or chorionic plate vessels) is associated with longer term and more severe infections
  • Commonly associated with ascending bacterial or fungal cervicovaginal flora infecting the amniotic fluid
  • Uncommonly associated with hematogenously disseminated bacteria (e.g., Listeria)
  • Associated with risk of neonatal sepsis and poor neonatal outcomes especially when there is a fetal inflammatory response
  • Histologic chorioamnionitis
  • Necrotizing chorioamnionitis
  • Chorionitis
  • Subchorionitis
  • Umbilical vein vasculitis
  • Umbilical artery vasculitis
  • Fetal vasculitis
ICD coding
  • ICD-10:
    • O41.12 - chorioamnionitis
    • O41.121 - chorioamnionitis, first trimester
    • O41.122 - chorioamnionitis, second trimester
    • O41.123 - chorioamnionitis, third trimester
    • O41.129 - chorioamnionitis, unspecified trimester
  • Extraplacental membranes: chorion or amnion (MIR)
  • Umbilical cord: vein or arteries (FIR)
  • Chorionic plate: chorion or amnion (MIR) and vessels (FIR)
  • Amniotic fluid infection by ascending cervicovaginal flora (common) or hematogenously or postprocedurally infecting organisms (uncommon) causes activation of material and fetal inflammatory responses
  • In the maternal inflammatory response, maternal neutrophils migrate from the decidual circulation into the extraplacental decidua, chorion and amnion, or the intervillous circulation into the chorionic plate and overlying amnion
  • In the fetal inflammatory response, fetal neutrophils extravasate from fetal vessels in the umbilical cord or chorionic plate and migrate toward the amniotic fluid (amniotropic)
  • Release of cytokines and other inflammatory mediators can lead to rupture of membrane or onset of labor
  • Reference: Pediatr Res 2022;91:289
  • Amniotic fluid infection is typically polymicrobial, often involving 2 or more organisms from vaginal or enteric flora; single pathogens may be seen in hematogenously disseminated infections or when 1 organism in a polymicrobial infection outcompetes the others
  • Organisms that are commonly isolated from amniotic fluid infections include (J Infect Dis 1988;157:113):
    • Ureaplasma urealyticum and Mycoplasma hominis (Clin Infect Dis 1993;17:S100)
    • Gram negative anaerobic vaginal flora (e.g., Bacteroides spp., Gardnerella sp.)
    • Group B Streptococcus
    • Peptostreptococcus spp.
    • Escherichia coli
    • Enterococci
    • Fusobacterium spp.
Clinical features
  • Clinical diagnosis (Obstet Gynecol 2016;127:426, J Perinat Med 2016;44:23):
    • Clinical chorioamnionitis (or intrauterine inflammation or infection) is diagnosed by a combination of physical examination findings and laboratory results
    • Isolated maternal fever:
      • Clinically documented fever ≥ 39.0 °C once or ≥ 38.0 °C (oral) twice
    • Suspected intrauterine inflammation or infection:
      • Fever (as above), plus 1 or more of the following:
        • Fetal tachycardia (greater than 160 beats per minute for 10 minutes or longer)
        • Elevated maternal white blood count (> 15,000 per mm3; in the absence of corticosteroids)
        • Purulent fluid from the cervical os
    • Confirmed intrauterine inflammation or infection:
      • Suspected intrauterine inflammation or infection findings, plus:
        • Positive amniotic fluid Gram stain or culture
        • Low amniotic fluid glucose (e.g., ≤ 14 mg/dL)
        • Elevated amniotic fluid white cell count (> 30 cells/mm3; in the absence of red blood cells indicating blood contamination)
        • Histopathologic evidence of acute chorioamnionitis
  • Positive:
    • Amniotic fluid Gram stain
    • Amniotic fluid culture
    • Placental tissue culture
    • Fetal tissue culture (e.g., lung tissue) in cases of fetal demise
    • Placental tissue (fresh or FFPE) molecular testing (e.g., PCR)
    • Neonatal blood cultures within day 1 of life
Prognostic factors
Case reports
Gross description
  • Dull, opaque membranes with yellow-green discoloration and cloudy amniotic fluid, possibly with purulent exudate
  • Can be grossly normal
  • Acute marginal hemorrhage in preterm deliveries (ISRN Obstet Gynecol 2012;2012:856971)
  • Multifocal umbilical cord surface microabscesses in C. albicans infections (APMIS 2018;126:570)
Gross images

Contributed by Drucilla J. Roberts, M.D.

Patchy surface, plaque-like lesions

Yellow / cloudy fetal surface of placenta

Microscopic (histologic) description
  • Acute chorioamnionitis should be staged and graded based on MIR and FIR (Arch Pathol Lab Med 2016;140:698, Roberts: Atlas of Placental Pathology, 2021)
  • MIR stage (location):
    • Stage 0 (preacute chorioamnionitis): neutrophils in the subchorial intervillous space beneath the chorionic plate (subchorionitis)
    • Stage 1 (early): neutrophils in chorion laeve of the extraplacental membranes (chorionitis)
    • Stage 2 (intermediate): neutrophils within chorionic or amnionic mesoderm
    • Stage 3 (advanced): stage 2, plus necrosis of amnionic epithelium or neutrophil necrosis
  • MIR grade (severity):
    • Grade 1 (mild to moderate): anything less than severe, as described below
    • Grade 2 (severe): confluent neutrophils or > 3 foci of > 200 neutrophils
  • FIR stage:
    • Stage 1 (early): fetal inflammatory cells within chorionic plate vessel walls (fetal vasculitis) or umbilical vein vessel wall (umbilical vein vasculitis)
    • Stage 2 (intermediate): fetal inflammatory cells within umbilical arteries (umbilical artery vasculitis) or vein
    • Stage 3 (advanced): necrotizing funisitis (perivascular bands of necrotic Wharton jelly containing dense neutrophils)
  • FIR grade:
    • Grade 1 (mild to moderate): anything less than severe, as described below
    • Grade 2 (severe): confluent fetal inflammatory cells with attenuation / degeneration of smooth muscle
  • Accompanying findings:
    • Acute intervillositis: aggregates of neutrophils in the intervillous space; often due to Listeria monocytogenes
    • Peripheral funisitis: wedge-like foci of neutrophils with necrosis at the periphery of the umbilical cord; often due to Candida
Microscopic (histologic) images

Contributed by Drucilla J. Roberts, M.D. and AFIP

MIR stages and grades

FIR stages and grades

Umbilical cord with surface abscess

GMS stain showing invasive fungal hyphae

Virtual slides

Images hosted on other servers:

17 week placenta with stage 2 grade 2 MIR

24 week gestation with clinical suspicion for acute chorioamnionitis with umbilical cord microabscesses

Positive stains
  • GMS (Candida species or most bacteria)
  • PASD (Candida species)
  • Gram (most bacteria)
Sample pathology report
  • Singleton placenta, delivery:
    • Acute chorioamnionitis (maternal stage X; grade X) with fetal vascular involvement (fetal stage X; grade X)
Differential diagnosis
  • Chronic chorioamnionitis:
    • Mononuclear infiltrate in the chorion laeve or chorion and amnion
    • Often associated with villitis of unknown etiology
    • Thought to be a host versus graft-like reaction
    • Has a recurrence risk
  • Meconium histiocytic infiltrate:
    • Meconium pigment within histiocytes in the membranes
    • Often accompanies acute chorioamnionitis
  • Acute inflammation in the space between the amnion and chorion:
    • Inflammation is not within the soft tissue or epithelium but in the space between the 2 membranes
    • Often associated with vernix caseosa or loose meconium
    • Not true acute chorioamnionitis unless the inflammatory cells are within tissue
  • Acute deciduitis:
    • Acute inflammation retained only within the decidua capsularis or parietalis, not in the chorion laeve epithelium
    • Feature of labor, not infection
Board review style question #1

A yellow / green discolored and cloudy placenta shows which of the following histologies of the membranes and umbilical cord?

  1. Stage 0 grade 1 acute chorioamnionitis, maternal inflammatory response with a fetal inflammatory response stage 1 grade 1
  2. Stage 1 grade 1 acute chorioamnionitis, maternal inflammatory response with a fetal inflammatory response stage 2 grade 1
  3. Stage 2 grade 2 acute chorioamnionitis, maternal inflammatory response with a fetal inflammatory response stage 2 grade 1
  4. Stage 3 grade 2 acute chorioamnionitis, maternal inflammatory response with a fetal inflammatory response stage 2 grade 2
Board review style answer #1
C. Stage 2 grade 2 acute chorioamnionitis, maternal inflammatory response with a fetal inflammatory response stage 2 grade 1

Comment here

Reference: Acute chorioamnionitis
Board review style question #2

This umbilical cord shows patchy white / yellow lesions on the surface. What is the most likely diagnosis?

  1. Candidal funisitis
  2. Fusobacterial funisitis
  3. Meconium myonecrosis
  4. Syphilitic funisitis
Board review style answer #2
A. Candidal funisitis

Comment here

Reference: Acute chorioamnionitis
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