Identification of Placental Mesenchymal Dysplasia and Twin Pregnancy With Complete Hydatidiform Mole: A Clinicopathological Analysis.

To investigate the clinicopathologic characteristics for differentiating placental mesenchymal dysplasia (PMD) from twin pregnancy with complete hydatidiform mole and coexisting fetus (CHMCF), a retrospective analysis was conducted on the clinical and pathologic data of 5 cases of CHMCF and 2 cases of PMD diagnosed in our department between April 2016 and December 2024, supplemented by a literature review. The microscopic features of PMD included stem villous hyperplasia with thick-walled stromal vessels, abundant villous stroma exhibiting edema and mucoid degeneration, an admixture of grape-like edematous villi and normal-appearing villi, and mild trophoblastic hyperplasia with pseudoinclusions. Immunohistochemically, PMD showed p57 positivity in villous trophoblasts but was negative in villous stromal cells. The microscopic characteristics of CHM are: hydropic swelling predominantly involving intermediate and terminal villi, often with central cistern formation, avascularity, and marked trophoblastic proliferation. Immunohistochemically, CHM demonstrated p57 negativity in both trophoblasts and villous stromal cells, along with high Ki-67 expression in trophoblasts. Placental mesenchymal dysplasia is a rare and often underdiagnosed placental abnormality associated with an increased risk of preterm birth and intrauterine fetal death. Its imaging appearance can resemble a hydatidiform mole, but its implications for maternal and fetal management and prognosis are vastly different. Therefore, accurate differentiation is crucial for appropriate patient counseling and clinical management. Clinicians and pathologists should be aware of this possibility.