Placenta

• Meconium passage in newborn infants is a physiologic event that normally occurs within the first 24 - 48 hours after birth
  • Meconium may be discharged shortly before delivery
    • Also considered a normal physiologic event related to gastrointestinal maturation
    • In this case, meconium may coat the membranes, fetal surface or cord but does not stain them
• Mechanism of intrauterine meconium passage is debated
  • Associated with increased motilin levels (Lancet 1979;2:968)
  • Controversy on its association with fetal distress
    • In utero hypoxia causes increased intestinal peristalsis and relaxed anal sphincter tone, resulting in meconium passage (Pediatr Clin North Am 1993;40:955)
    • Compression of the fetal head or umbilical cord (for example, in oligohydramnios) may cause a vagal response and meconium passage
• Minimum of 4 - 6 hours is required between the passage of meconium into the amniotic fluid and the staining of the placenta or fetus (Am J Obstet Gynecol 1975;121:45)

• Neonatal morbidity of all kinds has been significantly associated with meconium stained as compared to clear amniotic fluid (Kraus: Placental Pathology (Atlas of Nontumor Pathology), 1st Edition, 2005)
  • Associated with fetal acidemia, fetal hypoxia, clinical chorioamnionitis, low APGAR scores, higher rates of abnormal fetal heart tracings, etc. (Am J Obstet Gynecol 2023;228:S1158)
• Meconium aspiration syndrome
  • Seen in 2 - 5% of deliveries with meconium stained amniotic fluid
  • Defined as respiratory distress that cannot be otherwise explained in term neonates with meconium stained amniotic fluid (Am J Obstet Gynecol 2023;228:S1158)
  • Pathogenesis
    1. Meconium may cause airway obstruction → atelectasis
    2. Chemical effect of meconium may result in surfactant inactivation
    3. Local inflammatory response (pneumonitis) to meconium can lead to decreased pulmonary function
• Meconium associated umbilical cord vascular necrosis (Pediatr Dev Pathol 2016;19:315)
  • May be seen in cases of prolonged meconium exposure
  • Associated with vasocontraction and fetal hypoperfusion
  • May lead to adverse clinical outcomes and neurological impairments including cerebral palsy (Am J Obstet Gynecol 2005;192:452)

Contributed by Lucy Ma, M.D. and Joanna Chan, M.D.


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• Meconium laden macrophages
  • Phagocytic cells with fine, brown-yellow colored cytoplasmic granules, often vacuolated
  • May be present in amnion, chorion, decidua and umbilical cord stroma (in chronic cases)
  • Meconium pigment intensity on H&E stained slides significantly diminishes following exposure to fluorescent lights (Arch Pathol Lab Med 2003;127:711)
• Edema of the soft tissue plane between the amnion and chorion
• Degenerative changes of amniotic epithelium
  • Vacuolization of cytoplasm
  • Heaping up of cells
  • Loss of cells and necrosis
• Associated mild chorioamnionitis and umbilical phlebitis
  • Neutrophils in the amnion may be present as meconium promotes bacterial growth
  • Neutrophils in the wall of the umbilical vein
• Meconium associated umbilical vascular necrosis
  • Prolonged exposure to meconium
  • Associated with meconium laden macrophages
  • Necrosis of individual fibers of umbilical vascular smooth muscle
    • Rounding of myocytes
    • Marked eosinophilic cytoplasm
    • Pyknosis or loss of nuclei

Contributed by Lucy Ma, M.D.

A neonate and placenta are delivered vaginally at 40 weeks 4 days gestation. APGAR scores are 9/9. The placenta shows diffuse green staining of the membranes but is otherwise unremarkable. Microscopic examination of the fetal membranes would show which of the following changes?

1. Cysts that contain secretions and are lined by extravillous trophoblasts within the chorion
2. Diffuse infiltration of neutrophils in the amnion and chorion, including presence of microabscesses
3. Macrophages demonstrating yellow-brown granular pigment, which stains positive on Prussian blue
4. Vacuolization of the amniotic epithelium, heaping up of amniotic cells and edema of the amnion

D. Vacuolization of the amniotic epithelium, heaping up of amniotic cells and edema of the amnion. Green staining of the fetal membranes generally refers to meconium staining and the amnion epithelial changes listed are characteristic degenerative changes associated with meconium staining. Answer B is incorrect because it depicts severe acute chorioamnionitis. Answer C is incorrect because the yellow-brown granular pigments that stain positive for Prussian blue are indicative of hemosiderin pigment rather than meconium. Answer A is incorrect because it describes a pseudocyst (or chorionic cyst).

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Reference: Meconium staining

A 33 year old gravida 3 para 2 woman delivers a neonate at 39 weeks 6 days gestation. The pregnancy and delivery were uncomplicated. Gross examination of the placenta reveals a discoid placenta with a trimmed disc weight in the 75th percentile for gestational age, green stained slimy membranes and a left twisted umbilical cord with eccentric insertion. Microscopic findings of the fetal membranes are seen in the image above. Which of the following statements is true regarding this case?

1. Antibiotic therapy is the standard of care
2. It is predominantly associated with preterm births
3. Most cases with this finding have normal neonatal outcomes
4. There is a high likelihood of meconium aspiration syndrome

C. Most cases with this finding have normal neonatal outcomes. The gross description supports the meconium staining of fetal membranes shown in the image. There does not appear to be any other lesion noted in the placenta and most cases with meconium staining have normal outcomes. Answer B is incorrect because meconium staining is more common in postterm, rather than preterm, births. Answer A is incorrect because antibiotic therapy may be the management for chorioamnionitis or other infectious lesions. Answer D is incorrect because only ~2 - 5% of cases with meconium staining are associated with meconium aspiration syndrome.

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Reference: Meconium staining