Introduction: Neoplasms of the placenta are uncommon. Tumors arising from the placental tissue include two distinct histological types: the benign vascular tumor, chorangioma, and very rarely, choriocarcinoma. Benign leiomyomas, in contrast, are very common tumors of the uterine wall and occur in 0.1% to 12.5% of all pregnant women. However, the incorporation of uterine leiomyoma into the placenta is exceptional and raises the question of its origin. This case is possibly the first report on this kind of a placental tumor which has been examined using both immunohistochemistry and chromosome analysis.

Case presentation: A 34-year-old G4P3 Caucasian woman was followed up antenatally because of a stillbirth in her previous pregnancy. At 36 weeks’ gestation, a hypoechoic, 3.6 × 4.2 cm rounded mass was noted within the placenta on ultrasound examination. Histologically, the tumor was a benign leiomyoma and this finding was supported by immunohistochemistry. The newborn infant was male. Chromosomes of the neoplasm were studied by the fluorescence in situ hybridization technique and the tumor was found to carry XX chromosomes.

Conclusion: A rare benign smooth muscle neoplasm involving the placental parenchyma is presented. The tumor was a uterine leiomyoma of maternal origin, which had become entrapped by the placenta. This case report is of interest to the clinical specialty of obstetrics and gynecology and will advance our knowledge of the etiology of placental tumors.