Answer: Complete hydatidiform mole

Histology: The sections only contain chorionic villi. Fetal parts were not identified. There are exclusive markedly enlarged edematous villi showing central cisterns, and circumferential trophoblastic proliferation with marked cytologic atypia.

Discussion: This is a complete hydatidiform mole (CHM) with unusual long duration. It shows typical morphology characteristic for a CHM, including markedly enlarged edematous villi circumferential trophoblastic proliferation and marked cytologic atypia. In most cases, CHM is presented at earlier gestational weeks (<12 weeks), so called early CHM. The histological features of early CHM are subtle, including labyrinthine network of villous stromal canaliculi, hypercellular villous stroma with kayorrehexis, and some degree of trophoblastic atypia.

The differential diagnosis of CHM includes partial hydatidiform mole (PHM) and abnormal villous morphology due to other chromosomal abnormalities, such as trisomy or monosomy. In PHM, the villi are often comprised of two distinct populations of either small fibrotic or large hydropic. The latter displays irregular scalloped villous contour and some degree of circumferential trophoblatic proliferation. The villi in trisomy and monosomy may show some irregular trophoblastic projections and stromal trophoblastic inclusions. Trophoblastic atypia are usually mild or absent.

At presentation, the patient had an abnormally high serum HCG titer of 480,000 MIU/ML. Serum HCG is produced by syncytial trophoblasts and it is detectable one week post conception. Serum HCG peaks at around 12-14 gestational weeks to up to 300,000 MIU/ML and gradually decreases to below 55,000 MIU/ML in the 2nd and 3rd trimesters. Abnormally high level of serum HCG is one of the clues for suspecting a molar pregnancy. Serum HCG level rapidly decreases after removal of placental tissue, and is therefore an extremely useful marker in following up patients with molar pregnancies. In consideration of the potential risk of developing an invasive mole and choriocarcinoma, the current clinical management of complete hydatidiform mole includes dilatation and curettage, weekly serum HCG titer measurement (until obtaining 3 consecutive negative results), and contraception for at least 6 months thereafter.