Answer: Partial hydatidiform mole

Histology: The sections contain decidua and villi. No fetal parts are present. There are enlarged hydropic and small fibrotic villi. The enlarged villi show scalloped villous surface with proliferating trophoblast projecting randomly from the villous surface. In addition, cisterns and occasional trophoblastic inclusions are also noted.

Discussion: This is a well documented case of partial hydatidiform mole. Although the clinical presentation is nonspecific, the sections show typical morphology for partial mole. There are two distinct populations of chorionic villi, i.e., small fibrotic villi and enlarged hydropic villi. The latter displays irregular scalloped villous surfaces, cistern formation, stromal trophoblastic inclusions and remarkable trophoblastic proliferation. Karyotyping analysis revealed that the chromosomal make-up of the chorionic villi is 69 XXX, which confirms the diagnosis of partial hydatidiform mole. Partial mole can be very difficult to distinguish from early complete hydatidiform mole or atypical villous morphology associated with fetal monosomy or trisomy. The trophoblastic proliferation is quite pronounced in this case, give rise an initial impression of early complete mole. However, it lacks features of early complete mole, such as a labyrinthine network of villous stromal canaliculi, hypercellular villous stroma with kayorrhexia, and significant trophoblastic atypia.

On the other hand, abnormal villous morphology caused by fetal monosomy or trisomy often show less degree of trophoblastic proliferation and surface scalloping. Instead of two distinct populations of villi, most villi in monosomy or trisomy tend to be more fibrotic. In occasions when diagnosis can not be reached based on morphology, karyotyping, flow cytometry or FISH can be very useful in cases of partial mole due to its unique karyotype (triploidy). Partial moles are rarely followed by persistent gestational trophoblastic diseases, with a risk of 0.5-4.0%. Nevertheless, short-term clinical follow-up and serum HCG measurements are warranted.