Answer: Meconium staining

Histology: The sections of fetal membrane show features of expansion of subamnionic connective tissue by edema. In addition, there are numerous macrophages with cytoplasmic yellow-brown pigments in the subamnionic connective tissue, chorionic layer and decidua parietals. The findings are consistent with meconium staining. In addition, there is focal acute chorionitis (maternal inflammatory response, stage 1, grade 1).

Discussion: Meconium is fetal intestinal contents that first form in the early 2nd trimester. Although unlikely to be present in the amniotic cavity before 30 weeks’ gestation, it is seen in 1/5 of term placentas and up to 1/3 of post-term placentas. The finding of meconium-stained placenta has been associated with perinatal morbidity in small percentage of those newborns, which can be clinically manifested as low Apgar scores, low umbilical vein pH, respiratory distress, and/or seizures within 24 hours. In addition, meconium staining is often seen accompanying other more significant placental lesions, such as intrauterine infection and fetal thrombotic vasculopathy.

On gross examination of a meconium-stained placenta, the fetal surface, the fetal membranes, and the umbilical cord typically are discolored dark green. The membranes are commonly edematous, with a slimy consistency. Meconium can be identified histologically as irregular, non-refractile, yellow-brown cytoplasmic granules in macrophages. Those macrophages can be seen in superficial amniotic connective tissue after 1 hour of exposure and in chorion and decidua after 3 hours of exposure. The amniotic epithelium shows changes ranging from reactive to necrotic. The subamnionic connective tissue is expanded due to edema. A common differential diagnosis of meconium staining is hemosiderosis. The latter is produced by degraded blood products that diffuse out of hematomas within or adjacent to the placenta. Those hemosiderin granules have a more crystalline, refractile gold to yellow-brown appearance. In ambiguous cases, an iron stain may help in the distinction.