Third Trimester Stillbirth Associated With Hamartoma of Mature Cardiac Myocytes (HMCM)

Fetal primary cardiac tumors (FPCTs) are very rare. The majority of them correspond to cardiac rhabdomyomas, followed by other benign neoplasms or hamartomas. We describe the case of a third trimester female stillborn with an incidental autopsy finding of <i>Hamartoma of Mature Cardiac Myocytes (HMCM)</i>, a rare benign cardiac tumor previously unreported in the fetal or neonatal period. The intrauterine demise occurred at 32 + 6 weeks gestation after an uneventful pregnancy. The fetal autopsy revealed a structurally normal heart with a small subendocardial nodule just below the membranous septum. Microscopically, the nodule was well-demarcated from the surrounding penetrating bundle of the conduction axis and the adjacent left ventricular myocardium and consisted of disorganized mature cardiac myocytes in a haphazard arrangement with patchy mild interstitial fibrosis, consistent with HMCM. Awareness that HMCM can occur in the fetus is important in order to consider it among the differential diagnosis of FPCTs.